Specifically rackmount servers. According to their site, their slogan is "Helping you grow 1U at a time." Kek.
U is a measurement unit for rackmount equipment. There's a standard size for rackmount boxes. Each consists of a number of slots. A 1U box is the smallest unit for the rack enclosure, using 1 slot. A 2U rackmount box would be double the size of the first, using 2 slots.
My explanation was poorly worded. I should have just provided a picture.
I wish the format were more widely adopted. Could have allowed for a lot of space saving with appliances.
Here you go, champ.
Wait until you hear that I'm the one who edited it.