If you've never heard of the Pomodoro Technique then it's a method of improving your concentration based on - erm, a timer.
Focus Booster features a simple timer on a a black timeline, upon which a green progress bar gradually progresses. It's meant to be a more visual way of being able to keep track of your time although in effect, it's nothing more than a rectangular stop watch. The all-important options can be accessed by clicking the small and rather obscure 'i' on the right of the interface, where you can set the session and break time length. You can also choose whether you want it to tick or not.
The ticking does actually help remind you that you only have a limited time to complete a task, whereas the visual side seems a bit pointless although if you're listening to music or need the audio channels for something else, I see how it could be useful. Whether it will work actually improve your concentration depends on your mentality. It could quite easily work the other way and end up distracting you with its long (albeit resizeable) interface and annoying ticking.
If you really can't stick to the clock and need a gentle helping hand along the way to better concentration, Focus Booster is at least worth a try.