What does flashing a Honda Accord computer do?

The computer reads all sensor data and chooses the settings for fuel injector duration, ignition advance, cam set (in VTEC engines), and a whole host of things necessary to balance fuel efficiency and performance for the average driver. Most people are not "the average driver," though, so the decisions programmed into the computer are suboptimal for the type of driver you happen to be. By re-flashing your computer, you can (IF you get it right) improve performance or fuel efficiency or whatever is most important to you. It's a very tricky business though, so don't do it unless you have time to really book up or if you have another car at your disposal.

Good luck!