As others have said, yes the server already knows what it means, or which images are correct. However it is based on confidence, that that's how neural networks work. It's a statistical model to determine the right answer. So even though its already confident that "pictures 1, 3, and 8 have a bicycle", when you say that those images have bikes, you are reinforcing and training it further.

https://www.geeksforgeeks.org/machine-learning/what-is-reinforcement-learning/