The idea is quite simple, similar to Instapaper and the like. In fact, Google's own Chrome to Phone functionality has much the same concept: you're reading something in your computer's web browser, want to finish it on your phone, so you hit a button and up it pops on your handset. But, Read it Later Pro is rather more comprehensive than that. Naturally you can save links to read later, but the app will proactively download text and images so that you can read that content on a plane in the sky or a train in a tunnel.
On the app you're presented with options for whether text and images or only text should be downloaded, or indeed anything at all if you have complete and total faith in your data connection. You can also choose whether the integrated browser identifies itself as mobile or desktop -- handy for those who like scrolling. Sadly we couldn't get videos and other Flash content to show up within the app, but that's not much of a surprise given the app is called Read it Later, not Watch it Later.
Ultimately it's a handy app. The default "article view" does a good job of simplifying content for readability on smaller screens, and that you can send as many things as you like and read them whenever you like is certainly nice. Ground-breaking revolution in mobile reading? Nah. Worth .99? Sure -- if you like reading things.