In short
Pulling away from a stationary position on an incline without rolling back.
A hill start is the manoeuvre for pulling away on an incline without the car rolling backward. In a manual car: find clutch bite-point, apply gentle gas, then release the handbrake. In an automatic: brake-hold or hill-assist will keep the car still long enough to move foot to accelerator.
A small roll-back is a control fault. A significant roll-back is a serious fault, particularly with traffic behind.
See also