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. Your friend told you about her new boyfriend, but when you meet him you're surprised that he is more handsome than you expected. You say to her:

You didn't tell me how handsome he was.