what is it and whats it for?

Exhaust Gas Temperature.

its used for tuning, checking if your lean or rich, mostly used in a turbo application, tapped into the manifold.

1998 dodge avenger 4 cyl AT
Mods are:
Not enough

I use it to determine if I need to back off of throttle under boosted conditions. If it get's too high, chances are your pistons, rings, and other related components will start failing (cracking). You can sometimes determine a/f leaness or richness with it, but that is what the A/F guage is for.

-K-

It's a great gauge that I use secondary to an A/F gauge. The A/F is great for tuning, but when I'm running using nitrous I look at the EGT more to monitor the temp. of the exhaust gas.