It's a mode where you can't see your bar of star power and progress and crowd and multiscore
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HUD means Heads Up Display.
There is a theatre mode lobby in the main menu of multiplayer. You must enter it, choose a recent game, start it, and use the HUD on the bottom of the screen to help you edit it.
Because you must of pressed F1, to get your HUD back press F1
the hotbar
Sorry I didn't post the actual question, but when playing TF2 is there anyway to get different pictures of the classes in the HUD with the ammo and health?