The Design Rule Check is part of the PDK. Both the Magic and OpenLane tools make use of it. It contains rules that check things like: The gate of the MOSFET are the correct dimensions, Wires on the metal layers are not too narrow and not too close, Check that MOSFET gates are protected if they have long connecting wires - this is called an antenna rule. The design rules are generally a description of the smallest items that can be reliably manufactured so that a whole chip is functional.
