This is a failed evaporator core.
So this is how your air conditioner works.... The compressor is turned by the engine (or has an internal drive motor, for electrics and most hybrids), which circulates refrigerant (usually R134)
through the system.
Directly off the compressor, the refrigerant enters a condenser. The condenser looks like a thin radiator, it's purpose is to condense the pressurized refrigerant to a liquid state, by removing heat.
Depending on the type of system, there will also be a filter/dryer of some sort, to keep the system clean of debris and free of moisture. -- Depending on the type of system it can be before or
after the condenser.
The refrigerant then flows through either a thermostatic expansion valve or
orifice tube (depending on the system) which is designed to accept the high pressure liquid refrigerant on one side and disperse it as a vapor into the evaporator core. (see https://en.wikipedia.org/wiki/Venturi_effect
for info on how this makes cold)
The vapor flows through the evaporator, which itself is another radiator, of sorts. Heat from the air is absorbed through the evaporator and the the vapor, now charged with heat becomes gaseous and flows back to the compressor.... and in a nutshell, that's how your air conditioner works.Failure usually happens because....
When the air is cooled off flowing through, the evaporator also acts as an air dryer. The moisture that was being held by the warm air condenses on the cold evaporator. It builds up and flows out a drain that empties out of the bottom of your car.... but because of all this moisture, often times, over the years, the evaporator will corrode and eventually develop a leak. (See the closeup picture, and you get some idea)
These leaks are hard to accurately diagnose, without some doubt. The refrigerant can flow out of the smallest holes, and sometimes even specialized sniffing equipment can't detect the leak. And the evaporator can't be seen until it's removed.
One of the few ways I'm able to diagnose the leak is with my nose. Refrigerant has no inherent smell... But the oil that circulates through the a/c system has a very distinct smell, and usually I'm able to pick up on that. But I've been blessed with a very sensitive nose, and not everyone can pick up on the smell.
Notice on the picture below the small dark spot... That dark spot is from the oil that's leaked out of the system. It's not much.
Most of the time, the evaporator is a repair of last resort. When a technician has checked everything else for leaks, and can't locate anything visible under the hood in the engine bay(where everything but the evaporator is).