The U's stand for units whether it would be in 3Ds max/maya etc or even UDK. However first of all lets go through the basics of units in UDK.
16 generic units in 3ds max = 1 foot in udk
when you set your grid points correctly in 3ds max to match the sizing and snaps of udk, you will be able to create 1 foot with snaps enabled that equals 16 units.
So moving onto the image we see 64u which stands for 64 units within that floor section, this means that small floor piece is 4 foot wide (if you divide 64 by 16 you get 4). The avergae character scale is 96 units high which equals to the sum of 6 foot so you can pretty much visualize how big that small floor section will be.
However it does not stop there, expanding from the 64u section we have 128u and 256u which are the double of one another. When you create a plane to the size of 256 units, you can resize it uniformly to have of its value which leads you to 128 units and then 64 units, but this does not matter in level design as its just a representation of the perfect snapping to the grid. Basically in level design the grid is where all the magic happens, when you work on the grid with perfect snaps and power of two measurements, you will essentially be creating lego pieces that fit together in a level like in the image you posted.
I hope this helps a little and if theres anything else you may not know, inform us and well try our best to explain.
EDIT: oh and a good place to start for level design and unit size would be to look at modularity in games which you can read here: