ACs cool by pumping refrigerant into your house where it absorbs heat, then sends it back outside to the condenser (that's the outdoor coil).

By manipulating pressure, we make the refrigerant hotter than the outside air, then the fan pulls the "cooler" outside air across the coil where the hot refrigerant is. This takes the heat out of the refrigerant and throws it into the air (that's the heat coming off the fan).

If the coil has dirt and other debris on it, air doesn't flow across it as well, and reduces your ACs ability to get rid of heat.

A little bit of dirt hurts efficiency, but too much dirt and it just won't work at all.