1U stands for "One Unit" and refers to a piece of computer equipment that fits into a space that is 44.45mm (1.75") high.

This is a standard unit of measurement in the IT world.  Many of the servers that store data or run web sites are this height.

They are usually housed in a server cabinet or rack. Typically these cabinets can have the capacity to hold in the region of 40U i.e. 40 servers which are 44.45mm high.

Not all servers are 1U high, some are 2U or 4U high. In other words, they take up 88.9mm or 177.7mm in space respectively.

They are usually 19" wide.

Professional audio and video equipment usually comes with rack mount options and use the same measurement specifications.

Images courtesy of Apple