To defrost something is to thaw it, or bring it from a state of being frozen to room temperature. You can defrost frozen shrimp by running them under cool water for several minutes.
You have to defrost frozen food before you can eat it, and often before you can properly cook it. You might also need to defrost your icy windshield in the winter before you can drive your car. The word defrost comes from the Old English forst — before the 15th century, the words forst and frost were used interchangeably. When it's combined with de-, "off, away, or undo," it means "free of ice or frost."