Plain English Meaning
An MTR is a charted corridor of airspace where military jets are allowed to fly fast and low for training. Civilian pilots need to know where these routes are so they can watch out for fast-moving military traffic.
Full Definition
A defined route used by military aircraft to conduct low-altitude, high-speed training flights, often below 10,000 feet MSL and at speeds in excess of 250 knots. MTRs are jointly developed by the FAA and the Department of Defense and are depicted on aeronautical charts as IR (IFR) or VR (VFR) routes followed by an identifying number.