This thing here shows the health and resource (mana, energy etc) levels of you, your target and focus, and their targets. If you're a druid it will display your mana at all times, and if you're a death knight there will be extra bars for your runes.

To change the size of the bars, use /tutti hud options to bring up a menu.

You can also create custom bars with /tutti hud custom. This will give you another menu - fill in the fields and click Create. The value and max fields take input in the form of Lua snippets that must return a number - examples are "return GetTime()", "return UnitHealth('target')" or simply "return 5". The name field is not terribly important, but the Delete button will delete a bar with that name.