Catholics abstain from meat during Fridays during Lent, Ash Wednesday, and Good Friday. This is a form of fasting which is a form of penance. So, yes, it's a sin for Catholics to eat meat on a few Fridays of the year.
However, this applies only to people between 14 and 60 and excludes, the sick, elderly, and may exclude pregant and nursing women, and a few others groups under certain conditions.
More specifically, it refers to the meat of warm blooded animals (and not fish for example) and meat based soups, gravies, and that sort of thing, but not animal derivatives with "no meat taste" such as gelatin or butter.