Freezer bags do help prevent freezer burn on food.
However even when food is stored in freezer bags the food can still sometimes get freezer burn.
Freezer burn happens when the food gets air into the food while it's frozen so to prevent freezer burn on your food you want to keep as much air out of the food as possible.
So before sealing the freezer bag just seal it partially and then squeeze the air out of the freezer bag and then seal it.
Also wrap foods in plastic wrap or aluminum foil and then put it into the freezer bag or use freezer bags that you can remove the air from.
They make vacuum sealed bags that you can use a machine to vacuum the the air out of the bag which helps to keep the food safe from freezer burn and make it last longer as well.