What does a proof copy mean in 1880 Morgan silver dollar?

"Proof" means that the coin is in very good condition, and it could be worth $100-300, depending on who you sell it to. However, if it is a copy, you may as well toss it. The coin should say "copy" on it somewhere. If not, I would still check by looking up the weight of a real Morgan dollar and then weighing yours. Only a VERY good imitation can come close to the right weight.