Super Chip is a brand name for a tuner. Another popular one is the Diablo Sport brand.
A tuner is an electronic computer gadget that plugs into the diagnostic port of the vehicle and modifies the factory program that controls engine operation. Some of the things it does is change the ignition timing, alter the fuel delivery, etc.
The premise behind this is the believe the factory program is very conservative and there are potential gains available by altering the factory program. I use a Diablo Sport predator tuner, and I can say with complete certainty, it makes a difference. For me, it's a very good difference.
There are generally two types of tuners. I can't speak for the diablo, but I can speak for the Superchips and SCT tuners, because they are made for 2nd gen trucks.
First, the early Superchips (and Hypertech) are what is known as a "piggy back" tuners. Thes types of tuners modify your PCMs original parameters, but not everything. The changes can be noticiable, but are not radical. They are best for stock or very mildly modded trucks.
Other tuners, like the SCT, remove and store your PCMs original setup, store it, and relace it with a custom made one or a series of "canned" or preset tunes. The difference is that they leave none of the old programming in your factory PCM. As shipped by hemifever, the SCT allows for mod changes, so if you plan on doing more than just a catback exhaust and air intake, there are benefits. The SCT also has a better rep for removing the "death flash" and torque limiting programed into 1999 and later 2nd gen trucks.
Both units cost about the same, but used earlier Superchips can be found on ebay and elsewhere. They are not reprogramable like newer models, and you want to make sure they are not "VIN locked" to the last vehicle they were used on. In other words, the tune should have been removed from the vehicle and stored back in the tuner so it can be reused.