Safe socks are disposable plastic socks that can be obtained from the better bowling shoe rentals. They are mostly used by people who don't want to touch the inside of the shoes with their own socks or feet because of hygienic reasons.
Another popular use is during summertime when people come to a bowling lane only wearing sandals and realize upon arrival they can't wear bowling shoes without socks. Perhaps because it's forbidden at that lane but certainly because it's rather unpleasant and even painful after some time.
A pair usually costs less than a buck.