You may already know this, but if you have central air conditioning, then the evaporator coil is inside your furnace. They generally produce anywhere from some to a lot of condensate from the air in normal operation, depending on the relative humidity, so there's a capture tray underneath them to catch that water and drain it away so it doesn't rot your furnace. The tray has a drain that runs out of the furnace and into a nearby sewer drain if the furnace is in a basement, or somewhere outside otherwise. The drain line is small and the water flow is low and slow, so it often gets plugged up with unmentionable kukka like mold/mildew/algae/legionnaires disease, etc.
They're not precision things and aren't always installed with the greatest of care, so it's not unusual for there to be a bit of standing water in them, but it's usually more like a bit of a puddle than anything else. Typically, what doesn't drain will simply evaporate in relatively short order. A half-gallon in there would be a lot, indicating that the drain line is plugged.
Hopefully, he cleared that line for you, and told you how to keep it from getting plugged again.
