Downshifting is the act of moving appropriately to lower gears while slowing down. This is the essential difference between the operation of an automatic transmission and one of manual persuasion: downshifting not only helps you slow the car, but it also puts you in the right gear for the speed. Downshifting is great in bad weather or on hills, where immediate braking can be dangerous.
Keep in mind that you may shift down only one gear or simply apply the brakes. Again, knowing your range in each gear will help determine what's needed.
For example, if you are driving 45 mph in fourth gear and come upon a stop sign ahead, push in the clutch and shift down to third while using the brake; let the clutch out slowly to avoid high revs; repeat with second gear and then with first.