A trail, game run, or other track with only one ribbon of tread. But this is like defining an orgasm as a muscle cramp. Good singletrack is pure fun. Riding through loose gravel or sand, when the wheels slalom from side to side. A bike with front suspension has a shock-absorbing fork or stem.
Rear suspension absorbs shock between the rear wheel and frame.
A bike with both is said to be fully suspended. When a trail goes up a steep slope, it zigzags or switchbacks across the fall line to ease the gradient of the climb. These are rare, however, and cyclists often struggle through sharply angled, sloping switchbacks, which were designed for hikers and equestrians. Balancing on a bike in one place, without rolling forward appreciably. Cock the front wheel to one side and bring that pedal up to the 1 or 2 o'clock position.