ecu flash?

what the hell does it mean to have an ecu flash, ive heard mixed things...somebody clarify please as to how exactly you go about doing this...thanks~

Many modern ECUs use a form of reprogrammable memory to store the tables that control things such as fuel mapping, cam phasing, ignition timing, etc.... When we say the ECU is "flashed", we're talking about reprogramming the ECU and altering some of these tables, which optimizes the tune of the car.

The term "Flash" is being used in a generic sense here to describe the process of reprogramming a memory module. - VTEC.NET