But I'm still not clear about what it is exactly.

It can help you deploy to different cloud microservice platforms... AWS Lambda / Google Could Functions / MS Azure Functions

That's great. An abstraction for all of those that helps keep your code non platform-specific is very welcome.

BUT

They also charge a fee for bandwidth, instances etc...

So does that price cover the cost of the underlying service? Or is it a cost on top? OR is it a cost for their own platform (like, an option just like AWS or Azure, for which you can use Now for?).

thanks for any clarification!