There were three classes in Medieval society. The nobility comprised about three percent of the population. The clergy were another five to ten percent (depending on location). The rest, some ninety percent, were "the commoners." Not all the commoners were poor, a few percent were merchants and the like, some as rich as lords. About 10-15 percent of the commoners lived in towns and cities, while the rest them lived on, and worked, the land. Although it varied from country to country, about a quarter of the commers were serfs , one step up from slaves. The rest were free to one degree or another and many owned the land they worked.
Because of generally good (warm) weather from the 10th through the 13th Century , the lack of any major wars and improvements in farming technology , there had been something of a population explosion in Europe. As a result, by the early 14th Century, many farmers were working marginal land. Many others were working smaller farms than had their grandfathers. Thus there was less margin for error, and natural disaster. A few bad harvests and there would be substantial starvation deaths. This is exactly what happened in the first two decades of the 14th century, and notably so in 1315-1318, when a succession of bad years led to widespread famine in much of Europe. The bad weather varied from area to area, even within the same kingdom. But in the areas affected, the deaths amounted to ten percent, or more, of the population. There was little capability to bring in sufficient food when a large area had its crops fail, or, worst of all, two or three years of bad harvests.
Of coure, when times were good, life wasn't so bad. The commoners, even the poorest, ate an adequate (if rather bland) diet and drank a lot of ale, wine, and cider. The principal dish was porridge (mush). This was basically cooked grain, usually wheat or barely. This "main dish" was usually seasoned with a bit of salt and some vegetables (if available, peas were a favorite). Even the poorest had meat occasionally (usually pork) or fish (dried cod was a major item of trade). When in season, fruits, and vegetables were consumed. If money was available to grind the grain into flour, bread was popular, as was polenta and pasta --both essentially forms of bread-- in some areas. However not only was flour costly, but it spoiled easily in hot weather, while grain could be stored for years, hence the widespread popularity of porridge.
The "common people" were hardly common in terms of the huge number of skills they had to acquire just to survive. Although largely illiterate and unshcooled (in the formal sense), your average Medieval commoner spent his childhood and adolesence learning many practical skills. Most of these people lived on farms and to survive, and get ahead, had to know about crops, raising animals, and using (and repairig) a wide variety of tools. Most people lived in small villages, hamlets really, rarely containing more than a thousand people, and often less than a hundred. While there were some specialists for things like iron working, pottery and thatching, villagers had to possess among themselves the hundreds of different skills that made life possible, and perhaps a little enjoyable. Especially in France, Germany, and England, Winter was an annual catastrophe with which everyone had to be ready for, or face an unpleasant death. One could say that for most Europeans, life consisted of an annual preparation for the struggle with winter.
Commoners often built their own houses, and other structures. They made their own woolen cloth and garments of wool and leather. They planted and harvested a wide variety of crops, each of which required specific --and offten different-- knowledge. Half a dozen (or more) domestic animals had to be cared for (oxen, goats, pigs, chickens, beef cattle, sheep, geese, rabbits, etc.), each species having different needs. Clothing was usually home made, as was furniture and furnishings. If you liked to drink, and most common people did, you had to make your own alcoholic beverages (ale, wine, cider). Cheese was popular, because it would keep over the Winter. Pigs and other animals were slaughtered in the Fall and salted meats (like bacon) prepared for Winter use.
Entertainment was where you found it. News was eagerly sought from those passing through the village. There were no newspapers, books, TV or radio, so people talked to each other a lot. Their memories (not cluttered up with things like telephone numbers, traffic laws, and the like) were surprisingly good. Gossip was a favorite sport, as were the frequent sexual adventures and misadventures that kept the gossips busy. Among the many skills women learned from their elders (and are only now being re-discovered) was the knowledge of the medicinal use of plants. Certain potions could prevent pregnancy or induce abortion. Women probably lived in less fear of unwanted pregnancy in Medieval times than they did in the 19th century.
While there was not a lot of upward mobility for people who had to hustle just to survive, there was some. The nobles were always looking for bright and resourceful commoners to "enter service" in the aristocratic households. Many a young man or woman leveraged such opportunities into wildly successful careers. The church was also an avenue out of the village for the studious commoner child.
There was, after all, nothing very common about your Medieval commoner. These people managed to make much out of practically nothing and did so for century after century. We are all descended from them, or at least from those that were successful at staying alive.