In 600 CE, Europe had a population of approximately 14 million. By 1300 it was 74 million. That 500% increase was due to two simple changes: the methods by which agriculture operated and the ebb in large-scale violence brought about by the end of foreign invasions.
The first factor in the dramatic increase in population was the simple cessation of major invasions. With relative social stability, peasants were able to consistently plant and harvest crops and not see them devoured by hungry troops or see their fields trampled. Those invasions stopped because the Vikings went from being raiders to becoming members of settled European kingdoms, the Magyars likewise took over and settled in present-day Hungary, and the Saracens were beaten back by increasingly savvy southern-European kingdoms. Warfare between states in Europe remained nearly constant, and banditry still commonplace in the countryside, but it appears that the overall levels of violence, at least, did drop off over the course of the eleventh century.
Simultaneously, important changes were underway in agricultural technology. Early medieval farmers had literally scratched away at the soil with light plows, usually drawn by oxen or donkeys. Plows were like those used in ancient Rome: the weight of the plow was carried in a pole that went across the animal’s neck. Thus, if the load was too heavy the animal would simply suffocate. In turn, that meant that only relatively soft soils could be farmed, limiting the amount of land that could made arable.
A series of inventions led to dramatic changes. Someone (we have no way of knowing who) developed a new kind of collar for horses and oxen that rested on the shoulders of the animal and thus allowed it to draw much heavier loads, enabling the use of heavier plows. Those plows were called carruca: a plow capable of digging deeply into the soil and turning it over, bringing air into the topsoil and refreshing its mineral and nutrient content. Simultaneously, iron horseshoes became increasingly common, which dramatically increased the ability of horses to produce usable muscle power, and iron plowshares proved capable of digging through the soil with greater efficiency.
In addition to the increase in available animal power thanks to those innovations, farmers started to take advantage of new techniques that greatly increased the output of the fields themselves. Up to that point, European farmers tended to employ two-field crop rotation, planting a field while leaving another “fallow” to recover its fertility for the next year. This system was sustainable but limited the amount of crops that could be grown. Starting around 1000 CE, farmers became more systematic about employing three-field crop rotation: working with three linked fields, they would plant one with wheat, one either with legumes (peas, beans, lentils) or barley, and leave one fallow, allowing animals to graze on its weeds and leftover stalks from the last season, with their manure helping to fertilize the soil. After harvest, farmers would rotate: the fallow field would be planted with grain, the grain with legumes, and the legume field left fallow. This process dramatically enriched the soil by returning nutrients to it directly with the legumes or at least allowing it to naturally recover while it lay fallow. Thus, the overall yields of edible crops dramatically increased. Likewise, with the greater variety, the actual nutritional content of food became better.
Finally, starting in earnest in the eleventh and twelfth centuries, windmills and watermills became increasingly common for grinding grains into usable flour. The difference in speed between hand-grinding grain and using a mill was dramatic - it could take most of a day to grind enough flour to bake bread for a family, but a mill could grind fifty pounds of grain in less than a half hour. While peasants resented having to pay for access to mills (which were generally controlled by landowners, often nobles or the Church), the enormous increase in productivity meant that much more food was available overall. Thus, mills were still cost effective for peasants, and milled flour became the norm across most of Europe by the end of the twelfth century.
The medieval agricultural revolution had tremendous long-term consequences for peasants and, ultimately, for all of European society Thanks to the increase in animal power and the effects of crop rotation, existing fields became far more productive. Whole new areas were opened to cultivation, thanks to the ability of the carruca to cut through rocky soil As a result, there was a major expansion between 1000 – 1300 from the middle latitudes of Europe farther north and east, as the farming population took advantage of the new technology (and growing population) to clear and cultivate what had been forest, scrub, or swamp. In turn, the existence of a surplus encouraged lords to convert payment in kind (i.e. taxes and rents paid in actual foodstuffs and livestock) to cash rent. Likewise, the relative stability allowed smaller kingdoms to mint their own coins, and over the course of a century or so (c. 1000 – 1100) much of Europe became a cash economy rather than a barter economy. This gave peasants an added incentive to cultivate as much as possible.
Peasants actually did very well for themselves in these centuries; they were often able to bargain with their lords for stabilized rents, and a fairly prosperous class of landowning peasants emerged that enjoyed traditional rights vis-à-vis the nobility. Thus, the centuries between 1000 CE - 1300 CE were relatively good for many European peasants. Later centuries would be much harder for them. As an aside, it is important to bear in mind that the progressive view of history, namely the idea that "things always get better over time" is actually factually wrong for much of history, as reflected in the lives of peasants in Middle Ages and early modern period.