GUI

The GUI (Graphic User Interface) in Minecraft is what the Player views on the screen apart from the world. Some parts of the GUI in the game include the health bar, the hunger bar, and the experience bar. All parts of the GUI can be hidden by selecting in the Options menu.

Health Bar
The Health Bar is a GUI that displays the Player's health. Anything that makes the Player lose health, such as drowning or getting struck by a Hostile Mob, will make the hearts in the health bar go down. The Player will die when there are no hearts in the health bar.

Hunger Bar
The Hunger Bar displays the Player's hunger level. If it's equal to or over 9 shanks, the health bar will regenerate. If it is less than 3 shanks the Player won't be able to sprint. If it's empty the Player will start to lose health (This will only kill players in Hard Mode). The Hunger Bar will fill back up again if the Player eats food.

Experience Bar
The Experience Bar will track your levels that you have. It will fill up as the Player gets XP. If the Player fills up the bar, the Player will level up. Levels are used to do numerous things, like naming things in Anvils and Enchanting.

Hotbar/Inventory
The Hotbar allows the player to equip stuff from the Inventory. By pressing the button next to the Hotbar with the three circles will open up the full inventory. Here players can equip everything they have in their inventory as well as put on armor, or craft items that don't require a crafting table to craft.

Chat Button
By pressing the Chat Button, the Player will open up the Chat GUI. Here players can type out anything they want to say to others on servers or in their world.

Pause Button (iOS devices only)
The Pause Button will open up the menu for MCPE where the Player can see achievements, quit the game, or change options. Hitting the back button on tablets will do the same thing, but since some phones don't have that feature, they have a pause button.

Trivia

 * The pause button won't actually pause the game. Mobs and entities will still move.
 * However, there are mods that let the Player completely pause the game.