Hot bar

The hot bar is a selection bar which appears on the bottom of the screen. It is where you keep and select your items (Wood, obsidian, etc.). It is not your complete inventory, instead it is just a few slots of items that you can select.

The size of the hotbar can be modified by changing the "HUD Size" option in the video settings screen. Note this will effect all the other HUD's along with it.

Usage
If you want to put more items in your hot bar, simply open up your inventory (Defalt key: E, Y on X-box, the . . . Icon on pocket edition.), and then drag any item you want to your hot bar. The hot bar has a limited amount of items you can select. That depends on your device (computer & X-Box 360: 9 slots, iOS iPad/Android/XPERIA Play: 6 slots, View Sonic: 5 slots, iOS iPhone: 3 slots). The Hot bar acts like an "Extra slot" in your inventory, but with the ability to select items.