Dirty sock syndrome is the moldy smelling odor that comes from your air conditioner or heat pump when you first turn it on or when a heat pump goes into the defrost cycle. It smells similar to dirty gym socks which is where the name comes from. It is caused by mold and bacteria on the evaporator coil (indoor coil).
The best thing to do is to install a UV light to eliminate the odor and purify the air.