“5U” refers to the height of a module or rack format, with “U” being short for “rack unit.” A rack unit is a standard unit of measure for the height of equipment designed to be mounted in a 19-inch wide rack or a 23-inch wide rack. One rack unit is equal to 1.75 inches (44.45 mm) in height.
The 5U format means the module or rack is 8.75 inches (222.25 mm) tall, as it is five times the height of a single rack unit. The 5U size is commonly associated with the Moog Unit (MU) or Dotcom format, which is a standard for modular synthesizers influenced by the original Moog Modular systems.
Modules in a 5U format are larger than those found in Eurorack (3U) systems, and they typically use 1/4-inch connectors for audio and control voltage signals, as opposed to the 3.5mm connectors used in Eurorack. The larger panel size in 5U systems provides more space for knobs and patch points, which can make them more comfortable to use for some users.