it's the health race bar. You can read about it in the ABOUT.TXT which was distributed with your copy of nUI or on the download page at
http://www.wowinterface.com/download...-nUI.html#info -- it is rather redundant in the "Player/Target" and "Health/Power" HUD settings, but exceptionally useful in the "Simple HUD" mode and killer if you PvP. However, once you've read about it, if you choose to not use it, you can turn it off with the command '/nui hud healthrace'
I might suggest while you're reading the ABOUT.TXT (or documentation on the download page) you take a moment to skim over what other slash commands are available, as well. You'll also find a GUI tool written by Kodewulf on the optional files page (link given above) you can use to access all those settings from a panel.