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TDC = top dead center. There is a mark on the crank pulley that lines up with another mark on the block (?) when the crank is rotated so that cylinder 1 is at the top of its stroke (aka TDC). When the crank is in this position there are marks on the cam shafts that align with their marks. If the marks on the cams don't align when the crank is at TDC then the timing between the cam shafts and crank (and hence pistons) is off.

You can probably find a lot of info about this sort of stuff in threads dealing with replacing heads, installing timing belts, etc.
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