[P4g4nite]

The concept is simple enough. You want to make your engine more effiecient at pumping air, factory heads casting are generally rough, especially old models and this roughness makes a small, detrimental effect on air flow. So you grab a grinder and smooth out the passages.
You can also (if you have space) machine out the heads enough to fit larger valves.
You can 'deck the head', machine some height from the block to increase compresssion ratio.
You can also screw it up and make the engine less efficient than factory, make sure they have good experience at this thing before you pay them to ruin your motor.

What kind of engine is it? How much are you willing to spend? What other other modifications are you going to do to get the best out of headwork?

If you are just getting the head machined flat and aren't going to touch your intake or exhaust or stuff I wouldn't really bother wasting cash on expensive porting with dubious value.