Definitely an iced-up evaporator coil (the part inside the duct, usually above the furnace).
The quickest way to de-ice it is to go outside and unplug or turn off the disconnect beside the outdoor condenser unit. This stops the flow of refrigerant, which is the source of cold that's making the ice, while leaving the indoor blower motor running. The blower motor, in turn, will help circulate indoor air around and through the frozen evaporator, accelerating its melt.
The disconnect is often a gray box within a few feet of the outdoor unit which looks something like one of these:
There's a condensate drain line that comes away from the evaporator. It's probably dry at the moment. Some time after the outdoor electrical disconnect is turned off you'll see a trickle and then a steady stream of water exiting this drain. Eventually it'll slow to a trickle again, and then nothing, and at that point the evaporator coil is fully de-iced.
There are several common causes of evaporator icing. Several commenters have already identified that the refrigerant charge might be low. Another common cause is restricted air flow -- if you haven't checked or changed the air filter in a while, do so.
If the system continues to ice then you have basically three options:
- help the system limp along by manually intervening to de-ice at regular intervals,
- have the system serviced to bring its refrigerant charge up to spec, or to check for possible blockage at the orifice or TXV where refrigerant enters the evaporator coil, or
- round up the spare change from the couch cushions and piggy bank and get a new system installed.
Be warned that icing is not only inconvenient, but it can also lead to a burned-up blower motor in the furnace. That motor is cooled by the air flowing through the system, so when air can't flow, the motor overheats. It should have internal overheat protection, but repeated cycling and operating at high temperature will destroy the motor in short order.