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.
Sources of Symbols
It seems that almost any Scribus object can become a Symbol. Text frames, image frames, render frames, shapes, polygons, arcs, spirals, lines, Bezier lines, freehand lines, calligraphic lines, even PDF form checkboxes (and presumably other form elements) can all become Symbols. At least with a cursory trial, tables cannot become Symbols. An important thing to note about the conversion to a Symbol is that this cannot be reversed, in other words, you can't convert a Symbol into anything else. So if you want to convert an image frame into a Symbol, yet keep the original as an image frame, make a copy.
From the menu, choose Item > Convert > Symbol, or you can right-click the object for the context menu for the same result.
Editing a Symbol
A Symbol is a special object, or perhaps it's better to consider it not an object at all, in the Scribus sense. Your original item which you converted into a Symbol has some limited editing capabilities of that particular instance of the Symbol. These include the elements in the X,Y,Z tab of properties, and you may also create a drop shadow.
Additional editing is available from the Symbol dialog (from menu, Windows > Symbol). This will show all of your current Symbols from which you can for one thing, drag and drop to a page for another instance of your Symbol, or right-click and choose Edit item (or Remove item if you no longer want this Symbol).