I still cannot wrap my brain around this concept o

I still cannot wrap my brain around this concept of naked shorting...like how people do it....or is it only MM's and big companies that can do it? i understand shorting, how you sell something you borrowed, but how do you sell something you didn't borrow??? It may just be me, i am new to a lot of financial concepts, but learning everyday.

PS. not saying it doesn't happen, just hard for me to grasp the concept