Skimming is like it says. If the head is slightly warped, or you want to increase the compression, you can, within tolerances set by the manufacturer, skim very small amounts off the mating surface of the head (being the surface of the head which meets the block). By very small amounts we are talking thousands of an inch in some cases (dont know about the supra). If you take more of than recommended, you run the risk of the piston striking the valves at higher revs. If you rebuild and find the pistons strike the valves at all revs, you used an angle grinder