You should do read and follow the multiplayer tutorials.
You can consider the host to be the "real" game. Syncing refers to updating synced objects' properties (position, angle) on peers to match the host's game state.
Peers send their inputs to the host via input states or messages, and the host then applies those inputs to the game objects, which are then synced back to peers.
In a real time game, to hide latency, peers can also apply their inputs to their local copies immediately when pressed by using local input prediction.
Syncing objects and local input prediction are sometimes not necessary in a turn based game, given that results of inputs are deterministic. That is when the result of any particular input will always be the same on any device. Messages can work fine in this case for peers to communicate with the host either what was input, or the result of what happened after the input (or both). If not using synced objects, the host will then need to forward the same message to all the other peers. Naturally that doesn't apply if there are only 2 players involved.