Unstable means the broken ends of bone could come further apart under stress. The two ends of bone can even slide past each other causing what's called a displaced fracture.
Intertrochanteric refers to a place high up on the thigh bone. It's at the base of the femoral neck. The neck is a bridge of bone between the femur and the ball at the end of the femur that fits into the hip socket.
Intertrochanteric is one of two common hip fractures. Older patients in poor health and with poor function are most likely to have this type of break.