Monster frame is just another term for the starting frame of an encounter; because Cobalion is a stationary legendary, its monster frame is one, hence when you encounter it, the frame jumps up by one (for IVs). So to use a theoretical example, if you wanted Cobalion on a frame of 6, you'd advance 5 frames and then encounter it, as the actual action of going into battle with it will compensate for that last frame. If it's on frame 1, you don't need to do any frame advancements at all by that same principle.