What is it and what does it do? make your car Rev higher?

 

Depends on what you want to have done. A chipped ECU can do anything from handling boost (ie., the Hondata system) to adding a VTEC crossover to a non-VTEC ECU (such as the VTEC conversion available for B16A one-wire conversions).

Your general, run-of-the-mill chipped ECU will have a raised rev limiter, mildly massaged fuel & ignition maps, and probably a different VTEC engagement point.