Symbols are a new feature in 1.5.x versions of Scribus. Exactly what they are intended for is a bit unclear, but this wiki page will hopefully spur someone to add or supply some further information.
The name is a bit unfortunate, since in DTP we might most often associate it with symbol glyphs in a font. Experimentation with the Symbol feature shows it to have an association with Patterns, though there may be some other uses. What I want to do in this article is go over some things I have learned about usage.