The term has been around since the 1930s, and like all old slang that started out niche and went mainstream, it has been generalized and had its meaning diluted to the point of near-meaninglessness. There are readings of it where it and its opposite could describe the same thing, i.e. uncool and cool are the same. (Easy test to gage meaninglessness.)

I use it as a generic marker of approval or assent similar to "OK."

"We're moving the meeting to Tuesday."

"Cool."