
cool
Informal for very good, interesting, impressive, or fashionable.
Wow, dein neues Fahrrad ist echt cool!
Wow, your new bike is really cool!
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Informal for very good, interesting, impressive, or fashionable.
Wow, dein neues Fahrrad ist echt cool!
Wow, your new bike is really cool!

Informal for calm, composed, and unflustered, especially in a stressful situation.
Obwohl alle in Panik waren, blieb sie total cool.
Even though everyone was panicking, she stayed totally cool.

Pleasantly cool, refreshing. This meaning is less common and closer to the English original.
An heißen Tagen mag ich am liebsten einen coolen Drink im Schatten.
On hot days, I like a cool drink in the shade best.
The word 'cool' comes from English and belongs to informal language. It is very common in everyday life, but should be avoided in formal texts or very formal conversations.
Dein Vortrag war echt cool, gut gemacht!
Your presentation was really cool, well done!
You can recognize the meaning of 'cool' from the context. If it refers to a thing or idea, it means 'toll' (great/cool) (m01). If it describes a person's behavior in a stressful situation, it means 'gelassen' (calm/composed) (m02).
Ein Auto, das von alleine parkt? Das ist ja cool!
A car that parks by itself? That's really cool!
In the meaning 'gelassen' (calm/composed), the phrase 'cool bleiben' (to stay cool) is very common. It is often used as advice or an instruction to remain calm.
Bleib cool, wir finden eine Lösung für das Problem.
Stay cool, we'll find a solution to the problem.
great / mad
Toll and cool can both express a positive evaluation. Toll expresses general, open enthusiasm, whereas cool often sounds more casual and youthful and can additionally emphasize that something is modern or impressive.

Mein Freund findet den neuen Skateboardtrick cool.
My friend thinks the new skateboard trick is cool.
Meine Mutter findet den neuen Skateboardtrick toll.
My mother thinks the new skateboard trick is great.