it's (adjective) ("how" clause)

This phrase is used to talk about an idea, plus your opinion about the idea. The idea is something like:

That song caught on really quickly.

Then your opinion about it is something like:

It's surprising how quickly it caught on.

It's pretty interesting how quickly it caught on.

Some other examples of this:

I was surprised by how down-to-earth he was.

That's really cool how you made it look like they're flying.

This phrase appears in these lessons: