Bossbar

A boss bar is a bar that is shown when you use the  command, or for the wither, look at the wither's direction.

Showing a boss bar
A boss bar could be shown by using the  command.

Color
The boss bar has colors depending on which boss mob is active.

The ender dragon has a color of pink. The wither has a dark purple or pink boss bar.

ID
A boss bar has an ID, which can be used to do certain actions with one boss bar, including (but not limited to) disabling one. An ID is hidden, so you can't see it.

Disabling a boss bar
A boss bar can be disabled by using.