The water pan under the evaporator coil is most likely leaking, could be rusted out or the drain could be plugged. The coil is taking moisture out of the air and it supposed to leak but down a drain or what ever drain set up you have.
This it typical of all AC systems. Try to find the drain line coming off the coil and make sure it is clear or open. This may not be that easy to do if it is made of piping that is not see through. When the AC is running, water should be going through this line. Especially if it is a humid day.
If you are not familiar with what to do, I would call a mechanic ASAP to check it out. The water will eventually damage, your furnace or maybe your house.