The HUD or Heads-Up Display is information about the player which is placed over their view of the world. It details the user's health, hunger and experience bars, their item hotbar where players can quickly access items, as well as a crosshair which indicates which block or entity that the player is focused on.
The Minecraft HUD will automatically change depending on the situation the player finds themselves in. An armour bar appears above the health bar if the player is wearing armour, and the oxygen bar will display above the hunger bar if the player is underwater.
When riding a mob such as a pig or horse, players can see the mob’s health bar where their hunger used to be, which is a different texture to the players. If the mob can jump, like horses, the players experience bar is replaced by the mob’s jump bar, which details how much power is in the jump.
Any status effects inflicted on to the player, either via potions or mob attacks, are shown on the top right of the screen, going from top to bottom detailing which effect will run out first and flashing when they are about to expire.