From TF2 Wiki
Crouching is a feature that reduces the area of your character, making you a less visible target. This makes tasks such as defending sentry guns, sniping and ambushing easier. Crouch-Jumping may also allow you to reach certain areas faster, and also facilitates rocket jumping.
When crouching, the character will move a lot slower than the default movement speed. The default key for a crouch is "Ctrl" in the PC version and "X" in the Xbox 360 version. Crouching can be selected to toggle.
