Besides cooling the room, an air conditioner will always (slightly) dehumidify the air. In addition, some mobile air conditioners also have a separate dehumidification mode. The moisture extracted from the air will be collected in a water tank. This will fill up in due course and needs to be emptied. Even when the water tank is not yet full, the moisture may start leaking, for instance when you move the air conditioner.
You can empty the water tank by draining the water tank with the hose provided for that purpose. In addition to the cap, check that there is not also a plug on the drain point where you connect the hose.
The instruction booklet for your specific appliance has instructions on how to empty and drain the water tank.