That's really cool!

that's pronoun

— (rare, nonstandard) whose, of which

really Adverb

— When something is really a certain way, its very or very much that way. I really want to see a film. I'm really hungry now. You did a really good job on that. Thank you for coming and helping me this morning. I really appreciate it. I don't really like Japanese food. — If something really exists or really happens, it does exist or happen. Are you really going to the show? Yes, I really will be there.

really Interjection

— You can say "really?" when you are surprised. "He won yesterday." "Really?" — You can say "really" when you agree with something. "That girl talks about herself way too much." "Really. She's a nightmare."

cool Adjective

— If something is cool, its temperature is between warm and cold. It's hard to keep the house nice and cool on a hot summer's day. The air was cool, so he put on his jacket. — If you say something is cool you mean it is good and you like it. That's a really cool cell phone! Where did you get it? Wow, your computer looks cool!

cool Verb

— If you cool something, you decrease the temperature. Blow on the coffee to cool it off. — If something cools, its temperature decreases. The soup was too hot, so he waited for it to cool a little.