It means your broker will charge you that carry cost on a pro rata basis, assuming borrow also remains constant. (It's generally not a fixed rate.) So if you c

It means your broker will charge you that carry cost on a pro rata basis, assuming borrow also remains constant. (It's generally not a fixed rate.) So if you carry a 9mo short with 80% borrow rate on a stock that realizes 0 vol, you played yourself. If you don't want to deal with borrow, you can buy a put and sell a call on the same strike (usually slightly higher than ATM) which comes with an implied borrow rate that is locked in.