Since rollback netcode is shipping with all the latest fighting game releases and players are becoming increasingly demanding of it from developers, this thread will compile resources about it. @ me for stuff I should add to the OP.
What is rollback netcode?
Rollback netcode describes the implementation of a networking model for games to provide latency-free synchronized online multiplayer.
This system immediately runs the inputs of the local player (so that they are not delayed as with delay-based netcode), as if it were an offline game, and predicts the inputs of the remote player or players instead of waiting for them (assuming they will make the same input as the one in the previous tick). Once these remote inputs arrive (suppose, e.g., 45 ms later), the game can act in two ways: if the prediction is correct, the game continues as-is, in a totally continuous way; if the prediction was incorrect, the game state is reverted and gameplay continues from the corrected state, seen as a "jump" to the other player or players (equivalent to 45 ms, following the example).
Code Mystics explains netcode.
Often times when discussing rollback netcode, you will hear about GGPO or see the term used interchangeably. GGPO is the middleware that introduced the concept in the context of fighting games and is an essential piece of software to understand in its own right. Here are some high-level explanations of what rollback netcode is:
Where is it used?
How do I use it in my game?
There's a lot of approaches here so we'll start with the essentials to understanding the implementation.