It’s a “precept of the church” that Catholics are to abstain from meat on Fridays in remembrance of the crucifixion of Christ; since Vatican 2, Catholics have been told they can substitute another penance, but the stipulation remains, even if most Catholics ignore it except in Lent.
It is also traditional among Catholics to eat fish on Friday–not that one has to, as the precept is negative. One could observe the restriction by eating cheese pizza or Fettuccine Alfredo or pancakes or oatmeal or fruit soup. But Catholic tradition has said, “eat fish” (as if fish weren’t meat–i.e., the flesh of a living creature).
As a result of this tradition, fish fries are a staple of Lenten Fridays, hosted by the Knights of Columbus in many cases; in some parts of the country, restaurants have Friday fish specials. Here’s an article about fish fries in Wisconsin.
Now, if Fridays in Lent are really about abstinence and simplicity, it would seem that this tradition could best be kept by a simple vegetarian meal at home. It would seem that any simple meal at home would be more in keeping with the spirit than any purchased meal at a restaurant or church hall. And yet a Catholic could in good conscience have a lobster meal at an expensive restaurant and be in keeping with the church law while one who had a simple bowl of beef consommé at home would be in violation.