Split-system AC units produce condensation moisture on the indoor evaporator coil while removing humidity from inside the home. This moisture runs down the coil and into a drain pan located below the coil, where it is then diverted out to a drain (normally a floor drain, or into a condensate pump if no floor drain is available).
If water is leaking out from the edges of the coil, and onto and down the side of the furnace and onto the floor, chances are you have a plugged drain opening that is causing the drain pan to overflow. It’s also possible that your drain pan is cracked, or in the cases of older coils, rusted through. But well over 95% of these situations are merely caused by a plugged condensate drain, which is easily corrected.