The word "cool" means "interesting". It used to be slang, but it has become so common that most people don't consider it slang these days.
Although "cool" means "interesting", English speakers sometimes say "That's cool" about things that they don't think are very interesting. You just say it because you can't think of anything else to say. For example:
I had some eggs for breakfast.
Oh yeah? That's cool.
When you use "That's cool" in this way, the stress is on the word "that":
If you want to comment on something that really is interesting, say it with stress on "cool":