Perform a standard countermovement jump from a standing position trying to move as far forward as possible.
 
Stand with feet around hip width, brace the trunk, bend at the knees and hips to load the legs and swing the arms back, then swing the arms forward as you lean forward and push as aggressively as possible against the ground with the legs.
 
Once off the floor, lift and reach the feet in front of yourself to absorb the impact by bending the knees and hips. Reset completely for each subsequent rep.
 
Like with any jumping variation, you can increase the work for the legs and hips by eliminating the arm swing by keeping the hands loosely at the chest.  
Purpose
The broad jump is a basic exercise to develop explosive leg and hip extension that offers some variety from vertically oriented jumping and force absorption.
 
Programming
Broad jumps should generally be done for 3-5 reps per set, with 3-10 sets. In weightlifting, they are usually done at the end of a training session. This prevents their fatiguing the lifter for the performance of more sport-specific speed-dependent lifts, and also helps improve explosiveness by forcing the body to recruit more and higher-threshold motor units when partially fatigued.
 
Variations
Broad jumps can be done without a countermovement, with a full squat countermovement or from the bottom of a squat, or with one leg.