Hi!
I'm assuming this is coming from The Outsiders.
Greasers were the lower class, while Socs were the rich people.
The excerpt means a greaser can never be a socs, they'll always be a greaser. Likewise with the Socs.
But, those stuck in the middle get to choose whether they want to be a Greaser or a Soc.
It's a great book by the way!
Hope this helps fam!
~LocoCoco