The intercooler is a big mutha radiator (heat exchanger), that sits in front of the regular radiator, and cools the compressed air (from the tirbo) before it the intake manifold. This does two important things:

1. First, it cools the hot air closer to ambient, which raises the air's density. This means that more air is inducted into the combutions chamber (more zoom). The air is heated because of the compression process.
   
2. Second, because the air is cooler, the timing can be advanced a slight bit over otherwise non-intercooled air. Cooler air is less likely to induce detonation, so the slightly advanced spark and/or slightly higher intake pressures can give you more power (even more zoom.