End dump refers to how the trailer is emptied. The large, multi-stage cylinder at the font of the trailer is used to lift the front of the box until the material slides out the rear.
Tipping the box high enough to empty the load raises the center of gravity quite high. This unstable situation can reach a critical point on slanted or unstable ground that can ultimately cause the trailer to fall over
The advantage of end dumps is that they are very simple, both hydraulically and mechanically. This simplicity directly translates to lower maintenance and capital cost vs a live bottom or scrolling belt trailer.