How a Suspension System Works:
A suspension system is created with springs that absorb part of the shock when you hit a bump, allowing the tires and axle to move independently, and softening the impact to the rest of the truck.
If the truck's axle was attached directly to the frame, without any type of suspension springs, you'd feel every little crack in the road because nothing would be in place to absorb the impact. In fact, you wouldn't be able to control the truck, because its tires would bounce off the ground whenever you hit a bump.
Leaf Spring Suspension System:
A leaf spring suspension system is made up of one or more long, arched pieces of steel that are made to flex when necessary (like when you hit a bump or put a load in the truck bed) but with an ability to return to their original shape.
One end of a leaf spring is attached to the frame, and the other end is attached to a shackle that can move, allowing the spring's overall length to vary as its arch flexes (when carrying a load or traveling over bumps).