"Chipping" an ECU means replacing the program running the ECU. Chipping is just slang. What we actually do is program (or "burn") a new bin file to the ROM to replace the program currently running the ECU.
In order to chip an ECU, you will need the following:
Sounds pretty easy eh? Dave B. wrote a great Introduction To ECUChipping article that is an excellent place to start learning about chipping ECUs
NOTE: OBD0 MPFI ECUs do not always have a ROM for you to replace. Don't despair, Chipping An88-89 ECU is possible, too.