We are all familiar with our air conditioner being the large square box at the side or back of our house. However, your central air conditioner is a system with many components. Everyone knows their furnace or air handler is what heats them over the winter, however not everyone realizes the same blower motor that gives you heat in the winter also cycles your cool air in the summer. As you can see from the diagram, there is an evaporator coil above the furnace(normally hidden in your duct work). That evaporator is what the cold refrigerant is pumped through by the outdoor unit or condenser. Your furnace fan motor is forcing the warm air in your home across the cold evaporator coil in your duct work. The cold refrigerant absorbs the heat from your home and rejects it outside. If your furnace filter is dirty, there is less warm air blowing across your evaporator coil and therefore less cool air being cycled in your home. Likewise, if the outdoor condenser is dirty or covered by bushes, it is unable to reject as much heat from your home.