Minecraft wiki entry for Beginner%27s_guide, associated text: The Spacebar lets you jump; you can jump one-and-a-quarter blocks high, and can also jump across a two block gap in the ground (four blocks if sprinting). By default, walking into a one-block-higher edge automatically makes you jump up to the new level, but there are still many situations where you need to jump upward. If you turn off Auto-Jump you need to explicitly jump up to higher terrain. If you fall into water (or lava!), this same key is how you move upward toward the surface, and jump out onto the shore.