There are several meanings of "cool". Of course, there is the original meaning of "not hot". But other meanings include:

• popular
• interesting
• relaxed and not easily stressed

"Cool" started off as slang, but has been used for so long that it's OK to use in most situations. A normal adult can use the word "cool" in any of these situations:

• talking to a friend or family member
• in a business meeting
• talking to someone they just met
• in an email message
• in an advertisement

Situations in which it would be (just a little) strange to use "cool" are:

• if you were born before 1940
• in a written essay