When English speakers say "That's cool" in an informal context, it often means something like "That's great", "That's impressive" or "I like that". The closest Chinese equivalent would be something like "很棒" (hěn bàng), which translates to "very great" or "awesome." Something similar might be "很酷" (hěn kù), which directly translates to "cool," but if my memory serves me right also is used informally in Chinese to express admiration or approval.
It can also be used to convey admiration, similar to the Japanese "すごい" (sugoi).
But it is a word that carries a lot of weight, and has dozens of meanings in normal speech.
Technically, "Cool" in English can be a verb, a noun, and as an informal adjective it can have a wide range of meanings. And the meaning is often contextual. When used to describe another person, the range of meanings can vary between 'unfriendly' to 'calm' to 'self-controlled' to 'sophisticated'.
For example, Fancy Pants is cool. Because he's sophisticated.
>>3352737s
Rarity is cool. Because she's original and distinctive.
>>728285s
Rainbow Dash is cool. Because she's amazing and the most amazing of all the amazing.
>>2513s
But Trixie is not cool. Trixie is amazing.
>>390255s
Her absence of 'cool' is a part of her allure, for me. She is everything but cool. Except when she is cool.
>>829719s
Confusingly, Trixie can also be very cool, and often is very much 'すごい', which shows one of the differences between the English use of 'cool' and the Japanese use of 'すごい'. It is one of the ways in which words can have extremely narrow ranges of meaning in use, even when their definitions are almost completely wide open and undefined. Things can be very cool without being すごい. But also that makes it very cool.
>>390255s >>2320969s >>68861s
This is now a Trixie thread. Which is both cool and すごい.
>>105656s
But - at the same time - Cranky Doodle Donkey gives Pinkie Pie a cool response, because he just wants to be cool. And she's not being cool about it. Which is not cool.
>>73912s
It was cool in the end though, because Cranky and Matilda got back together, which was cool.
>>90765s
They were 'chill'. If only Pinkie could learn how to 'chill' more sometimes, it would be cool. But that's what makes her a cool character - she can't keep her cool.
I hope this helps, but 'cool' is a really great example of a word that 'means what it means when it is used' and can mean something completely different just a second later.
If you have an example of the use of 'cool' in a sentence that you'd like broken down and explained (hopefully - I'm just a translator, not a linguist) let me know. You can ask in 中國人 if you'd like, but I really don't have much experience with it (I'm just green-book and industry trained) so I don't know any of the colloquialism. But if you know 日本人, that might be an easier meeting ground for some of the more obscure uses of the word 'cool'.