What does it actually mean for a COIN to be considered a SECURITY?

Okay, so we know it's bad obviously, due to the current market condition.

But what does it actually mean exactly? Could someone here explain this in layman's terms?

Like what does it mean for the average user, for the exchanges, and for the future of crypto if they ever considered as a security?
Also, is it safe to expect the same thing from other countries? from authorities similar to the SEC (If any)?

I'm trying to wrap this around my head, to know what to expect in the future.

If any POS coin is a security, and any POW coin is a threat to the climate (although we know its BS) then what are we left with here?