The coils are where the cooling takes place. As such, moisture from the atmosphere condenses and falls into the condensate drip pan. That pan has a drain line which feeds by gravity into a condensate pump. The pump (like a sump pump) will pump the water out as it reaches a critical level. Older pans were metal and did rust -- eventually needing to be replaced. Modern pans are plastic and don't.
If you have standing water under the coils, you have a problem with your condensate line or the pump itself. Clearing the drain line and checking the condensate pump is essential.
Does that help you? Hope so.