40mm Dynamic Drivers
The drivers are what allow headphones to function. They are hardware devices (not to be confused with software drivers that you’d find on Windows, for example) designed to translate electrical signals into vibrations that our ears can hear. Speakers, in other words. When a driver is “dynamic” that just means that it moves and is actually the most common manufacturing choice.
The 40-millimeter part describes the size of those drivers: Generally, the larger the drivers, the more powerful and clear the sound, which is why Apple mentioned it. 40mm is a good size for headphones – you can find professional and gaming headsets with bigger drivers, but this is certainly on the larger end and promises good things about audio performance and spatial features.