UDim stands for Universal Dimension, and uses 2 coordinates. A UDim consists of a Scale and an Offset. These are used to figure out where exactly the UDim's position is.
Scale is a number between 0.0 and 1.0 that is a percentage of the parent object's size. For example, if the parent object is 50 pixels wide, a UDim scale of 0.5 would be 25 pixels wide. 0.5 makes the size cover 50% of its parent, same as 0.38 is going to cover 38%, 0.85 is going to cover 85%, and so on.
Offset is a specified Integer. This is a non changing value that is factored into the UDim calculation.
The final result is a combination of Scale and Offset. Scale * Parent's Size + Offset.
Source: ROBLOX Wiki - http://wiki.roblox.com/index.php?title=UDim