Noclip, also known as fly and ghost, is a cheat that disables the effect of gravity and the effect of world solids on you. This allows you to glide freely through the world. Although this is a good way of drawing nav_ meshes, if you are playing on a server then as you may notice your crosshairs are as far out as they can go. If you exit the world or simply fly into a wall you'll not be able to shoot. You cannot die. Entities such as point_hurt will not effect you.