After you've read the link that Christophe suggested...
Spring Web Flow, in short, is a way of building flow-ful web applications...that is web applications that follow a predefined path(s). A typical example is the checkout on an e-commerce site, but there are more interesting examples if you look hard enough.
You can build flow applications without Spring Web Flow. In fact, I've seen them built in Struts. But when you do that, you typically spread the flow across multiple actions/controllers. There's no one place that describes the flow.
Spring Web Flow promotes the flow as a first class citizen of the application. You can go to one or more flow definition files and get a pretty good idea of what the flow looks like. The actions (which are usually just method on some bean somewhere) have no idea where they fit within a flow or that they're even part of a flow.