Freezer burn happens when moisture in the outer layers of the food evaporates into the freezer air, leaving behind empty "dry" pockets in the tissue of the food. This is technically a process of chemical sublimation and is actually done intentionally when making freeze-dried foods.
Freezer burn is visible on meat as brownish-white discolorations and on other foods as puckered white splotches. While it's not harmful to eat, freezer burn adversely affects flavor and texture - as you've probably experienced! Unfortunately, there's no way to reverse freezer burn and affected portions should be removed before cooking.
To prevent freezer burn, you want to reduce the food's exposure to air. Wrap large items like portions of meat and loaves of bread in a double layer of plastic wrap, wax paper, or aluminum foil, then store in a container or plastic freezer bag. Store smaller items like zucchini slices or liquids in portion-sized containers or plastic bags so the food fills the container entirely with minimal leftover space.
This won't prevent freezer burn entirely, however. Foods that have been frozen for extended periods of time will inevitably begin to develop freezer burn. For this reason, try to rotate your freezer contents so you eat the oldest foods first.