When frost builds up to the point where it causes problems, one needs to remove the frost to return the system back to normal operation. Probably the two most common items that need defrosting are vehicle windows and old freezers.
During cold weather, moisture in the atmosphere comes in contact with the cold glass of vehicles. This causes a small ice crystal to form. As time progresses, the ice builds up into a sheet. One can manually scrape away the ice or, if nobody is in a hurry, one can turn their rear window defroster on plus turn on their heater while the car is running and switch the blowing air to come out of small vents in the dashboard. This raises the temperature of the glass until the ice melted and it flowed away.
In ancient times when there were no frost-free freezers, when the freezer became overfilled with ice one had to defrost it manually. I would get a saucepan filled with boiling water and set it in the freezer, repeating as the water cooled down. Eventually large icebergs would dislodge and I could grab a large chunk and toss it into the sink to melt. When the freezer was cleared of frost it could be turned back on and filled with food. Defrosting would happen around every three months unless the seal around the door was compromised, in which case one would have to defrost it more often.