A dialogue reset is when you reload the level during audio dialogue. The most common application is switching characters at a save point while cop dialogue is happening. The result is, if you reload the level early enough, the dialogue restarting from the very beginning. You can do this infinite times, as long as you don't let the dialogue finish.
Dialogue resetting is essential to all versions of Heli Skip, as well as older Hikage routes (it's still used for the optimal English route).