As is an adverb that's usually used to compare one thing to another. You could say, for example, that your cat is as smart as your sister.
When you're noting similarities between two things or people, especially when you talk about degree or amount, you'll find yourself using the common adverb as. You might complain that it's as hot as heck, or express disappointment that your homemade casserole doesn't taste as good as when your mom makes it. As comes from the Old English alswā, "similarly."