In the context of filesystems, 8.3 refers to the DOS days, particularly the FAT filesystem (used on floppies and hard drives), having a table of files that reserved 8 characters for the the filename and 3 in the extension.
Many other filesystems, and most filesystems since then, do not make the filename/extension split part of the filesystem, so do not have separate limits (most filesystems still have a 255-character overall limit for each filename/directory name)
Microsoft later added long filenames (LFN), which associated longer filenames with existing 8.3 entries.
The way they do this is basically a hack onto the same FAT table, one that older tools should ignore, while most newer tools would only show the longer filename.
If you see filenames like FILENA~1.TXT you know the longer filenames got lost.