The verb "caption" has several meanings, depending on its context.
1) In photography or filmmaking, to caption refers to providing a written description or explanation accompanying an image or video. This is usually displayed at the bottom of the screen for subtitles in videos or printed below photographs in magazines and newspapers.
2) In literature, to caption means to provide a title or heading for something such as a book, article, song, etc., that summarizes its content or theme.
3) In art history, to caption refers to providing an explanation of the subject matter in a painting, drawing, sculpture, etc. This is usually displayed beside or below the work of art.