"Cool" has 3 meanings:
1- When talking about something that can be touched or felt, it means "a little cold".
example: "It's cool outside.", means it's a little cold outside, but not too cold.
2- When talking about feelings, it means "calm" or "relaxed".
example: "Don't worry, and keep cool."
3- When talking about how someone or something is (especially in Spoken or Informal English), it means "excellent", "fashionable" or generally "great".
example: "That dude got a very cool car."
See the 1st example which is "It's cool outside.", can have the meaning of no. 3 too, it can also mean "It's great outside, why are you staying in?".