Chipping is re-mapping the ECU on a fuel injected car (the thingy that controls when the injectors fire, etc). The standard chip program is often a bit wooly to take into account different altitudes, emissions regs, crap fuel, etc, so a re-map just optimises it.
It's a bit hit and miss I find. On my two last cars it varied. I had an E36 BMW 325i which it made a big difference too, and then an old bimmer 535 which it didn't do that much too (largely to the extremely restrictive exhaust system, methinks). So, fingers crossed. The Preza already has a filter and pipe which helps maginify any effects of the chip.
I just like chipping things I think - my mobike is chipped, my old psx was chipped, we're thinking of getting the cat chipped :!: and I'm trying to find a ps2 chip that doesn't involve 5 hours with a soldering iron to install.
Chipping is righteous, chipping is good.