Lint/dust/pollen build up and need to be cleaned out. Have you checked the owner's manual? If you don't have it, you can probably find it online. It probably shows how to take the cover off and get to the drain.
Sometimes, the drain line gets kinked. You need to check it out to verify. There is a sloped pan underneath the evaporator coil to collect the condensation. At the low point, is the drain hole.Sometimes a pipe cleaner or a wire or something can be used to poke through it and clear out whatever is plugging it.