Freezer burn is generally the result of non-airtight packaging. Freezer burn is a condition that occurs when ice crystals form in an area of the package where air is present. As water molecules escape to the surface the product underneath is dehydrated and becomes dry and brown or gray with a leathery appearance. Fats will oxidize and have a rancid flavor. This process can take days to weeks depending on the quality of the packaging and the temperature of the freezer.
Freezer burn can be prevented by using proper moisture and vapor proof packaging materials and wrapping tightly to prevent air pockets where water can leach out of the product and collect on the surface.
In addition to proper packaging freezer temperature plays an important part in the prevention of freezer burn. Temperatures of 0 deg F or below are necessary to minimize freezer burn. Fluctuating temperatures can also be a source of problems. As the temperature fluctuates above 0, moisture loss from evaporation can occur.
Food that is freezer burned is not necessarily unsafe. If the product was frozen the entire time until thawing for use it will be safe to use. However, it is likely that there will be an off or rancid flavor so freezer burned areas should be trimmed off and discarded before preparing.