rackspace : 2u
i don't understand that statement

This means how many "spaces in a rack" a server will take up. Do you have a 2 U server?

If you are looking at a Colo Provider then think of them as real estate managers, what they are selling is 'space to mount your server' and 'bandwidth'.

The 'U' is the unit of computation of the height of the server. If I am not wrong, 1U is 1.75" and 2U would be 3.5" in height.

A rack is split up into 49U or so typically. Typically, it's how they calculate space use. There are servers that range from 1U to 2U and so on. So it depends very much on the servers you have that you want colocated. Typically 1U to 2U means that your server has to be rackmounted. Otherwise, it won't fit. So you may want to consider that as well (as in the form factor of your server if it's existing or if you are buying one)