When you want to talk about the level or amount of a quality, you use the phrase "how ___ something is". For example, if it's been raining a lot and the ground is really wet, you can say:

I can't believe how wet it is.

Another example is when you meet someone who's famous and seems unfriendly, but he is actually a nice guy when you meet him:

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