I know that Unity doesn't generally ask for any royalties but will I be asked to pay for anything if I develop an app on Android (pay to Android)?
It seems to be saying on their website that Android is NOT a free software?
Android is intentionally and explicitly an open-source -- as opposed to free software -- effort: a group of organizations with shared needs has pooled resources to collaborate on a single implementation of a shared product.
No cost. They mean that Android is not free to companies like Motorola, HTC, etc. who implement it on their phones. You can do what you like with your Android apps. If you sell it through an app store, they'll want a cut, but you could sell it from your own web site if you wanted to. (People tend to trust lone-wolf sites a lot less).
There's a one-time $25 developer fee if you want to distribute on the Android Market.
There is no cost at all. You can do what you like with your Android apps.