The UV Editor allows you to quickly select UV co-ordinates for UI components if the default (0, 1) settings are not correct. The most common use for this tool is when UI widgets are using atlas textures instead of individual textures.
The drop down list in the top left hand corner allows you to switch between the assigned texture to the UI widget or the atlas texture of the UI Element. Hit the refresh button next to it if you want to reload the texture at anytime.
In the next four boxes are the UV co-ordinates for the top left and bottom right points of the UV sample area. These can be adjusted manually or by using the mouse on any of the four control points in the preview window. By clicking and dragging a control point you can select the part of the texture you want displayed and the UV coordinates will automatically be calculated.
Clicking and dragging anywhere within the selected area will move the selected area to a desired position. Note that you can also inverse the selection, which will be visually displayed by selection area turning red.
Additional helper buttons can located at the top of the editor which reset the selection area (), flip the selection are horizontally (), and flip the selection area vertically ().
Once the desired UV coordinates have been specified, simply hit the Select button in the top right hand corner, and these changes will be applied to the UI widget.