Drone is a resonant noise from your exhaust system. Typically it will sound similar to sitting in the cabin of an OLD truck with oversize tires while cruising down the highway. LOUD hum.
It can be mild & slightly irritating, making it difficult to carry on a conversation around town driving. It can also be loud enough or at the right frequency to give you a headache if you drive for an hour or two - making it unpleasant to drive your car.
Ideally, you should get a nice rumble at idle & when you start your car, and a nice deep but quiet tone when cruising on the highway. And of course the system should come alive & put a smile on your face every time you plant your right foot. I had an SVT exhaust on my V6 SE and replaced the center resonator with a small 5x8 Magnaflow. THAT system was NICE! Did all the above, but may have been too quiet for some. I prefer to be more subtle. Magnaflow makes a 4x7 muffler now that would be a nice replacement for the center res of an SVT exhaust (5x8 is a bit tiiiiiight), but I don't know if the tone would be deep enough...