Stockfish is a computer program that can play chess against humans or other computer programs. It's just like a brain inside a computer that knows how to play chess and makes moves without getting tired.
Imagine you have a toy chess set and each piece is controlled by a tiny robot. You tell the robots to move the pieces and they calculate the best moves, just like Stockfish. But Stockfish is much smarter and faster than those tiny robots because it can calculate millions of moves in just a few seconds.
Stockfish has a big book of all the known chess moves and strategies, and when it plays against someone or something, it looks at all the possible moves and picks the best one. It can also learn from past games and adjust its strategies to be even better.
So when you play against Stockfish, you can be sure it will make the best possible moves, just like a person who knows chess really well would.