In a nutshell, it's the time it takes to charge the ignition coil in between fires. With a distributor type ignition, there's a charge delay between each pulse sent to the spark plug through the wire. This means you need to properly adjust dwell time when going with larger injectors, plugs, plug wires, or altering fuel pressure because your ignition dwell time can and will change. If you do not compensate, the ignitor will be forced to discharge before it has a chance to fully charge, which gives you less than optimum spark characteristics and then you don't have an optimum burn cycle in the combustion chamber.
This is why most people go with an aftermarket ignition system rather than actually fix the problem (which 80% of the time is related to ignition/injector tuning.)
Jamie's cars are proof that even stock distributors can handle more than what most people put to the wheels without issue.