SEC will enforce all sorts of regulations, both on the people behind the security (think Vitalik and the Ethereum foundation in the case of ETH) and any company that provides trading on that security (basically, every exchange out there that is not Bitcoin only).

These regulations put a lot of constraints on what's possible and what not by the participants, as well as many audit and transparency mechanisms in place. Basically, it turns the wild west of cryptos into a space as regulated as a normal stock market (this is, for those who comply with authority, of course).

Depending on how the SEC approaches, it could mean the death of many shitcoins and a massive slow-down to the shitcoin casinos.