A Client Prediction multiplayer game model.
Let GameLogic be an implementation of the game's working logic, and let lc and ls be instances of the game logic from the client and server respectively. Then, upon any input, the client will execute this input with lc and send that input to the server, which will simulate the same input with ls.
The server will then periodically (every ~33 ms in this code) send every client the game state (including player positions, velocities, states, etc.) as determined by ls. Then the client will 'replace' its own game state with the server's by backtracking its buffer of game states (sometimes a practice of interpolation is used instead). This is called reconciliation. In between the reconciliations, lc is still running which allows the client to render its current game state (essentially predicting the next game state based on the previous reconciliation).