Variables are used for quite a few different things. One thing they do is store data.
For example, if you insert a text entry box, Captivate automatically creates a variable to store whatever the user types into that box. You can then use that in an Advanced Action, or insert the variable into a text caption or shape as a kind of placeholder--when published/previewed, the strange looking $$variableName$$ turns into whatever was typed into the text box.
Another use of a user variable is to keep track of something behind the scenes. A common, basic show/hide Advanced Action uses a variable to keep track of whether or not an object is hidden or visible, for example.
There are also many, many Captivate system variables. Some are "read only," meaning you can't change their value, while others you can change with an advanced action to change Captivate's behavior (such as cpCmndShowPlaybar, which you can use to show or hide the playbar).
You cannot attach an Advanced Action to something static like a caption--it must have some sort of "event" that triggers the action, like clicking a button or entering a slide. Lilybiri has a good blog post on that: http://blog.lilybiri.com/events-and-advanced-actions
Basically variables can be used to do a great number of things, especially when combined with Advanced Actions. It's hard to cover it all! Image may be NSFW.
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