Having strong core muscles (abdominals, lower back and obliques) helps to hold you upright, gives you good posture and protects your back against injury. Good core strength helps you do things as simple as walking.
Crunches are a simple and effective abdominal exercise to strengthen your core.
How to do crunches:
Lie on your back with knees bent and feet flat on floor. Place hands at the sides of your temples and tighten your abdominal muscles (pull your belly-button in). As you breathe out, lift your head and shoulders off floor, keeping your lower back on floor. Hold this position for a moment and, breathing in, lower your shoulders and head to the start position.
Aim for 10-16 crunches, then repeat this 2 or 3 times.
As with all exercises, if you feel any pain during this exercise, stop immediately.
Also see my post about back extensions, which should be done at the same time to ensure you are balanced at the front and back of your body.