In a nut shell...watch the included video (with download) or click on their site to watch it. It gives a good explanation.
This free app does what many wished was built in...if you are looking at a web page, reading something, watching a movie or whatever....Screebl will keep your screen lit up (I am not talking about when you are touching the screen...I am talking about when you are just looking at something). When you put your phone in an unusual position such has completely horizontal, it allows your phone to shut off like it typically would. Whats nice about this is two fold...first....you no longer get the annoying dark screen all of a sudden when reading or looking at something. Yes, you could set your phone to not dim for a few minutes, but that would kill your battery if you set it down. With this, you can set your phone to go off at 15 seconds, but it will still stay lit up when you are using it.
Second...it could actually save battery life overall by using the 15 second time out.
There are a lot of different settings that can be changed, but like the video will say, most of the defaults are good. They do recommend, and so do I, setting up the vertical and horizontal postioning of your phone based on how you hold it. This is how it knows if you are looking at something.
Very nice and well worth a look....