Mark mines is a special move command in which an engineer unit operates on a minefield to make it much less dangerous to infantry. Only infantry benefit from minefield marking. Antitank minefields can be marked, but this doesn't change their danger to vehicles.
An order to mark mines is only allowed on a known minefield which is not already marked. Also, it is only allowed when the location of the engineer unit, or its last waypoint, is close to the known minefield. ("Close" here means in the same action spot, or in any of the eight adjacent action spots.)
A unit performing a mark mines move will sit still for two or three minutes, not even turning to face the direction of the mines. Then, at the start of a turn, you will notice that the minefield's warning sign turned from red to light yellow; in that same turn, the engineer will crawl onto the now-tamed minefield.