Well, it's sort of an in-progress feature. You can use it, but it doesn't do everything we want it to yet.
Configurations are just sort of tweaked versions of your project. So you could, for example, have two configurations: 'English' and 'French'. If you create the French config by copying the English, then go through and translate all the text objects to French, now you have two versions of your project in one - the French and English versions - and you can switch between them with the Configs bar.
However it's pretty limited and I don't think it's actually particularly useful yet, it's something we want to expand on in future.