It contains all the UI elements in the game, including the main canvas, configured in HUD and Menus gameObject.
Inside of it, you can see where the dynamic elements of the game are located (icons that follow positions of objects on-screen or that are disabled/enabled to show info about anything needed).
The gameObject HUD contains the main info of the game screen, including the info of the player (health bar, stamina bar, weapons HUD, abilities info, etc…), the in-game menus, like inventory, map, mission log, experience/skills menus, etc…
After that, you can see the touch panels which contain the different touch panels and their buttons for mobile devices.
And finally, the pause menus, inside Menus gameObject, along with the settings, die, and exit from the game menu.