This is likely a slight typo or a specific variation of the H.265 (HEVC) codec.
If you meant this as a , you might use: You (2018) - S01E01 - Pilot [h255].mkv
You – S01 (h255) Format: WEB-DL / x265 Season: 1 Release Tag: h255 (internal encode group / repack version)
The suffix in the filename indicates a specific video encoding release tag. In the world of digital video distribution and piracy, these tags are used to identify the technical quality and compression method of the file.