(Replay engineer): we basically record all the inputs and outputs to a program. In the example of an HTTP request: when recording we'd record that a request was made, and the response. When replaying, rather than make the HTTP request, we return the response that was recorded.
If you're interested you can learn more about how Replay works here: https://medium.com/replay-io/how-replay-works-5c9c29580c58