The Align command on the Texture menu takes the bounding box of the selected UV vertices and moves all the vertices in the specified Direction to the line in the box indicated by the Align To option.
The options in the Align UVs dialog include:
Direction: Moves the vertices in the specified direction. Both Up and Down move the vertices in the V direction. Both Left and Right move vertices in the U direction.
Align to: Determines how to align the vertices in relation to the bounding box.
Low is the minimum value of the box.
Average is the average of all the values.
Center is the center of the bounding box.
High is the max value of the box.
The UV Tools toolbar also has buttons to quickly align UVs up, down, left, or right.
