"Decking" the heads is basically resurfacing the area of the cylinder heads where they meet the block to make sure everything is straight and true. This is usually done when a head is removed due to heat warping to ensure and good gasket seal between the head and the engine block.
Decking/resurfacing, because it removes a minute piece of material from the heads, increase the compression ratio ever so slightly. Will it give you more power? Yes. Will you feel it? Most likley not.