Modern people in the West have an uneasy relationship with meat. On the one hand, we love it. The smell of hamburgers on the grill or turkey roasting gets everyone's attention. It's obvious from our teeth and from archaeology that our Stone Age ancestors ate meat (normally cooked) as part of their diet. Meat has more calories per volume than vegetables (generally), so if one is hunting and gathering it makes sense to hunt for meat. Humans can eat pretty much every kind of animal, whereas an awful lot of plants are indigestible or just don't produce nutrients for us (grass won't work for humans).
On the other hand, there are a lot of things we think "bad" about meat. Every recipe for a "healthy diet" starts by talking about cutting back on red meat with its saturated fat. Many vegetarians and vegans believe it is not right to raise sentient creatures just to kill and eat them. (I once saw a sign in range-country Wyoming, fighting back: "Did you know that every day thousands of innocent plants are killed by vegetarians? Eat more beef!") This is further complicated by the popularity of things like gluten-free diets, the so-called "Paleo" diet (not clear where Paleolithic woman would have gotten the olive oil), and low-carb diets, where one tucks happily into meat.
As I discussed in an earlier post on the medieval diet, medieval people had a lot fewer foods to choose from than we do, because they didn't have New World foods like corn and chocolate (much less processed foods or fruits and vegetables out of season). For them, the majority of their daily calories came from bread. "Give us this day our daily bread" was not spoken metaphorically. However, they loved meat and ate it when they could. They couldn't have cared less about saturated fat.
Meat was expensive, rare, and hard to keep fresh centuries before refrigeration. For most medieval people, pork was the most common meat, but it was only eaten fresh in the fall, at the time of the big pig roundup and slaughter. Everyone at their fill, then the rest was smoked and salted to last the winter as an occasional treat.
The aristocracy hunted for deer and went hawking for birds, but over-hunting then, as now, can drastically reduce the population of hunted creatures, so the powerful tried (without overwhelming success) to restrict the right to hunt to themselves.
For most people, beef was eaten only when the old dairy cow was no longer producing, and chicken when the hen stopped laying, though young bulls and cockerels might be harvested out of the flock. Songbirds and rabbits provided additional sources of meat.
Red meat was considered to make one lively and lusty. Monks considered this bad, so they normally ate no red meat at all. (It all made sense in terms of Humors.) To lead a pure and simple life, breaking away from material things, they had to give up meat. Meat broth might be allowed, however, if a monk were ill, and there were always stories (about other monasteries, of course) about monks who feigned almost constant illness so they could live in the infirmary and have meat.
For a special guest (like a bishop or a great lord), the abbot would be expected to serve something better than the bread and vegetables on which the monks normally lived (with maybe some eggs). So they would serve fish or maybe cheese (but preferably fish), not red meat but a little classier than regular monastic fare.
This idea that fish was a semi-penitential food, halfway to being a vegetable and certainly not to be considered red meat, gave rise to the assumption that Catholics should eat fish rather than meat on Fridays during Lent. (It used to be every Friday, but it's just been Fridays-in-Lent since the 1960s.) These days, however, eating fish (generally more expensive than hamburger) has lost a lot of its penitential aspect and seems more a ritual act.
Our grocery was advertising "crab legs for Lent." Let's not go into detail on that one.