For some video files, separate subtitle files are available. These are usually .ass, .ssa, .srt or .sup files or .sub and .idx file pairs that you can just put in the same directory with the same base name as the video file, and they will be displayed when the video is played. Most general use video players are able to load these subtitles automatically out of the box.
If the subtitle file is for just one episode, you should add the file to the correct episode. If it is a file that contains many subtitle files for many episodes, say in a RAR or ZIP archive, you should add the file to the first relevant episode and then add file-episode relations for every other episode that that archive has subtitles for. In this case, extract the archive and add each included subtitle file to its relevant episode entry as well.
Adding subtitle files is rather similar to adding video files; the file does not of course get any video or audio tracks generated, but the normal rules for the release group, release date, CRC verification (rarely used for subtitles), version numbering and subtitle track language apply. Be sure to read and follow the special guidelines for quality and source fields, though.
In many cases, these separate subtitle files have been designed to be used on a certain, usually raw, release. To link the subtitle together with its intended raw video file, simply add a file relation between the two with the New File Relation button on the file page.