Hold position is a StarCraft and StarCraft II command, usable by any mobile unit. In StarCraft, worker units lack this command, can be assigned to hold position if non-worker units are in control simultaneously. To assign units to hold position, press "H".
Units holding position will not move to engage enemies or chase them down. It is an essential way to prevent hit and run tactics from luring entire groups of units. In StarCraft II, units will not move aside to allow units pass - this includes allied units and those controlled by you.