Air conditioners remove moisture from the air. In this process, water collects in the base of the unit.
Normally this water evaporates, however, some water may sit stagnant at the base of the air conditioner for a long time.
This problem is inevitable however can be reduced by cleaning the base of the inside of the air conditioner at least once a year.
Regular cleaning prevents dirt, linen or dust from absorbing the water and allowing mold or mildew to grow. Replacing the filter behind or in front of the cover helps.