Unit Frames are, quite literally, the frames that display unit information.
The default unit frames are the ones in the top left corner that display portrait, health bar, power bar, etc.
While they display the information correctly, their style and position do not lend them selves to being able to quickly read the information.
For addons that replace the default unit frames look here:
http://www.wowinterface.com/downloads/cat21.html