An ECU or Engine Control Unit controls your engine by either by referring to preprogrammed air/fuel maps in its memory or by reading data from a series of sensors. It then takes this data and along with other preprogramed maps it makes up its own A/F map.

When a ECU starts up after a reset it it takes readings from the engines sensors and makes up a table or a map of the fuel, air temp etc. From then on it refers to this table to provide the correct fuel, timing, air amount etc to the engine so when you e.g. open up the air intake more air is flowing into system. The ECU doesn't know about this extra air so it doesn't provide the extra fuel needed to make the power out of it.

When ever you do simple mods to a stock engine in anyway, it would be a good idea to reset the ECU.

The best way to reset your ECU is by disconnecting the negative terminal from you battery and then pressing down on your brake pedal for about 20 seconds, this will drain out any residual current.

After you reconnect your battery, take her out on the freeway and do a few WOT runs in every gear, this way if you added a better air filter the ECU will detect the extra air and compensate by adding extra fuel.

Then you can reset your radios channels :)