Guard rails keep you safe on your deck. The point is to keep people including young children from falling from the deck. Guards are required for decks higher than 30 inches from the ground. In some municipalities, a guard is required for a deck that is 24 inches from the ground. The specifics of a quality guard rail are spelled out in local building codes but this list will get you started:

The guard rail should be 36 inches high (a deck over 6 feet high requires a 42 inch guard).
There should be no openings larger than 4 inches so nobody can fall through.
It should be strong enough to hold a person that falls heavily into the rail or balusters.
It should not be easily climbable - no footholds.