Just as aerobic activities challenge the heart and lungs and strength-training challenges the muscles, stretching challenges the tendons and ligaments (along with the muscles) so that your body becomes more supple all around and you increase your range of motion. That makes injuries less likely to occur—and thereby makes you less likely to have to interrupt an exercise program with days of rest.
You should engage in five minutes of stretching, or flexibility training, after every exercise session. (You don't want to stretch before you exercise because if you're not warmed up, you'll risk "snapping" your "rubberbands" before they're "loose.")
Each stretch should be performed twice, with slow and deliberate movements. And each stretch should be held for 20 to 30 seconds. It may produce a little discomfort at first, but it feels good after you've been doing it a while.
Good stretches include the hamstring stretch, the cross-over stretch for your buttocks and hips, the quadriceps stretch, and the shoulder stretch.