A subtitle is the text at the bottom of the screen in a movie. The purpose of a subtitle is usually to translate the movie dialogue from a language the viewer doesn't understand to another. Subtitles can also be useful for people who for some reason have trouble following the dialogue in a movie. Personally, I mostly use them to watch movies that are originally in English (I prefer originals over translations, that are often inadequate), which is not my native language and it can be a problem for me to understand it sometimes by hearing only.
Watching DivX movies with subtitles
Today, literally every movie player supports subtitles in sub files, so you do not need to worry much. Just download subtitle file and plase it into folder with the movie, change its name so it is the same as the movie file name except for the extention part (leave it .sub) and it will be picked up by your movie player automatically. Or you may select the subtitle file from your movie player manually. Refer to your movie player help section in order how to do that.