VMU (Minimum Unstick Speed) is the lowest speed, at which the aircraft still is able to take off. Because the speed is so low, the angle of attack (the "nose up") has to be higher, thus the tail is lower and touches the ground. Below VMU an aircraft is not able to take off, it would skid along the runway, the angle of attack is not big enough to produce enough lift to get airborn, above VMU the aircraft is able to take off without the tail touching the concrete (but tail strike could still occur due to overrotation).
Dani