Tail wind is a term used to describe the movement of air coming from a direction in movement with an aircraft. In other word, if an aircraft is flying in a Northerly direction with a tail wind the wind is blowing in a northern direction so that the wind moving hits the back of the plane first and the front of the plane last. Planes taking-off will use the runway in opposition to the tailwind as to have a headwind, which is more favorable for takeoff.
A tail wind can present many problems to a plane. When a plane flies it moves through the air at a certain speed and the air moves past the plane and under the wings at a certain speed. At takeoff before the plane starts to move if there is a tailwind, the wind is moving under the wings but in a negative direction causing the plane to, once it starts moving, feel like it is moving slower. This forces the plane to work harder. For example: If you are running on a treadmill at 5mph you do not move because the ground in moving in the opposite direction at 5 mph. if you speed up to 6 mph and leave the treadmill at the same speed then you will start to move forward on the treadmill at the rate of about 1mph. This is how a tail wind works. If the plane has a 5 knot tailwind and it begins its roll, when the plane reaches 5 knots in speed it will act as if it’s moving 0 knots.
Tailwinds are also present in microburst, which are a leading cause in airplane disasters. They can force an airplane to increase their rotate speed (lift-off speed) and tailwind can also force and aircraft coming in to land to increase their landing speed. When in actual cruise flight, it can slow the airplanes true airspeed (which is the speed of the plane through the air).