MRL refers to the initials imprinted on the bottom tread of Master Chief's boot in Halo: Combat Evolved. They are the initials of Marcus R. Lehto, Halo's Art Director. Lehto was responsible for Master Chief's skin and armor design.
An easy way to see the initials is if you're on a reflective surface, such as Two Betrayals while on the Control Room's bridge, look down on the floor and on the Master Chief's boot are the initials. An alternative way is to play co-operatively and have one player crouch while the other zooms in on their right foot with a pistol or sniper rifle.