The rear of a taildragger is closer to the ground than the front. As the aircraft moves forward (I guess at its stall speed), its tail rises up and the aircraft moves with the fuselage level before takeoff (As seen here). But why? Does the pilot change the elevator?
Yes, when the plane exceeds a certain speed (known to the pilot!), the pilot then pushes the stick slightly forward and this lifts the tail off the ground. This reduces rolling drag and sets up the angle of attack of the wing for takeoff. For very powerful airplanes, the tail will sometimes come up on its own and surprise the pilot during the takeoff roll.
In some planes, the resulting angle of attack is enough that the plane will fly itself off the ground and begin climbing when it has picked up enough speed in the roll. In other designs, the pilot will wait for the right speed and then gently pull back on the stick to increase the AoA a bit and the plane then takes off.
Think of it as a wheel barrow, with the center of gravity behind the forward set front wheels.
The weight x distance of the CG behind the front wheels (when the aircraft is at rest) creates a torque force holding the tail down. As the plane begins it take-off roll, the tail and wings begin to counter-act the weight torque with lift, raising the tail off the ground. In the video, the pilot "helped" it a bit with some down elevator (increases tail lift coefficient), easing off as speed increased.
It is important to note, with proper technique and design, the plane should not be at an angle of attack to fly off at the lowest possible airspeed during the take-off run.
Just like a tricycle gear, angle of attack should be held lower until the aircraft is moving fast enough to lift off safely even in the event of a change in wind. The concept of rotation to fly-off is the same, around 1.2 Vstall for a recreational aircraft.
In some tailwheel aircraft, at some trim settings, if the pilot exerts no forward or aft pressure on the control stick or yoke, the tailwheel will rise off the ground as the aircraft picks up speed during the take-off run, and if the pilot continues to exert no forward or aft pressure on the control stick or yoke, the aircraft will rise into the air at a reasonable flying speed at an angle-of-attack that is safe for normal conditions. After the tail is up, if the pilot wishes to attain a higher than normal takeoff speed, he may exert forward pressure on the stick or yoke to keep the main wheels on the ground for an extended time, so that the plane eventually lifts off at a rather low angle-of-attack. Conversely, by exerting aft pressure on the control stick or yoke, the pilot can make the plane lift off of the ground from the "three-point" attitude, so that all three wheels leave the ground at the same time. This maneuver should be approached with caution due to the high angle-of-attack involved, but it is safe in some aircraft in smooth conditions.
As to why the tailwheel rises off the ground if the pilot exerts no forward or aft pressure on the control stick or yoke-- it is basically because the plane is trimmed to fly at a much lower angle-of-attack than the angle-of-attack that exists when the aircraft is travelling horizontally along the ground in the "three-point" attitude, with all three wheels on the ground. As the weight on the wheels decreases during the takeoff run, the aerodynamic forces become more and more important in comparison to the forces exerted by the wheels, and the plane tends to adopt an angle-of-attack close to the angle-of-attack that it is trimmed for.