A safety is a mechanical device incorporated into disposable lighters designed to keep young children from setting themselves on fire. They also function to make your cigarette (or what have you) as difficult and frustrating to light as possible. No smoker buys a lighter with a safety built into it if they can possibly help it.
Safeties generally exist in two designs, one more aggravating than the other. The first is the trigger-stop method - a small piece of plastic is fitted beneath the lighter's fuel trigger that prevents it from being depressed without manually lifting the stop out of the way, usually by lifting a small lever with a thumbnail. The problem with this design is, if the fuel didn't ignite the first time through, you're forced to depress the lever every time you attempt to spark the lighter. This is frustrating to the extreme.
The second design, most commonly seen on Bic brand lighters and their ilk (the rounded lighters, not the square ones) consists of a strip of metal that guards the flint wheel, requiring more effort than a child can produce to strike a spark. No spark, no fire.
Safeties aren't nearly as prevalent as they used to be, but it's worth noting that, while they haven't disappeared completely, their design has changed slightly so that their removal has been made trivial, at least to an adult - most safeties, especially of the flint-wheel protection design, can be popped off with a housekey with minimal effort.