TL;DR

Enhanced Bars is a specialized AddOn that improves main Blizzard UI health bars with DoT, HoT and Absorb informations. It was originally released in Mists of Pandaria and has been updated for BFA.

Details

Enhanced Bars displays on the health bar:

The part that will be lost because of temporary DoTs.
The part that will be recovered thanks to temporary HoTs.
The power of active damage absorption effects.
The power of active healing absorption effects.

Each information is coded by a specific color (see picture).

In addition, Enhanced Bars will show a red draining animation when you take damage and a yellowish-green recovery animation when you are healed. This can be used to better visualize burst damage or healing.

Enhanced Bars has been powered-up for Legion (patch 7.0) and now works with nameplates!

Blizzard absorb bar is disabled if you are using Enhanced Bars absorb bar, to avoid duplicating informations on the health bar.

BFA introduced PvP Scaling to reduce gear power gaps. Enhanced Bars is aware of this mechanic and is taking it in account to have DoT/HoT indicators as accurate as possible.

What is enhanced

Player health bar
Pet health bar
Target health bar
Target of target health bar
Focus health bar
Target of focus health bar
Party health bars
Arena health bars
Battleground health bars (flag/orb carriers)
Nameplates

What is NOT enhanced

Raid health bars
Boss health bars

Over Time Predictor

Enhanced Bars is bundled with Over Time Predictor AddOn. This AddOn computes DoT, HoT and Absorb values on players and mobs. If you disable or remove this AddOn, then Enhanced Bars will no longer show DoT and HoT values on health bars.

Advanced / Customization

Enhanced Bars is intended to be an extremely lightweight, enable-and-play AddOn and thus provides no graphical interface to customize its features, mainly due to the fact there's not many things to customize in the first place.

However, confident users can easily choose which features to enable (DoT, HoT, absorb) and customize colors through very simple modifications of the Options.lua file.

Enhanced Bars comes out of the box with sensible default values, so most users shouldn't have to worry about this.