Rocket Jumping is a powerful tool that will propel you at quick speeds and to great heights.
The simplest Rocket Jump consists of equipping the rocket launcher and then looking down, jumping, and firing at the same time. This will cause the rocket to explode below you, and the explosion will send you flying. You can also Rocket Jump off walls, which can give you enormous speed boosts when combined with Strafe Jumping.
You can use this technique to reach great heights or cross large gaps that would normally be too large. But be careful! Your rocket explosions will hurt you too, and kill you if your health is low!