Refers to modules that are 5U (rack units) or 8.75” (222 mm) high, which is most often associated with the vintage Moog standard and those who have followed in their footsteps, including Synthesizers.com (Dotcom) and Moon Modular. You will sometimes hear this used interchangeably with MU for Moog Units, which also refers to a standardized width of 2.125” (54 mm) wide per MU. Given that this standard is both historical and physically large, some users “5U” as a badge of honor that they’re traditional and cool. (And the are.)
There was also a briefly popular 5U format from MOTM that used a different width and power connection. It has since been discontinued, but there are still diehard MOTM format users today.