Plain English Meaning
A list of letters you put on your flight plan to tell ATC what gear your aircraft has — what radios, what navigation equipment, and what kind of transponder. ATC uses that list to decide which routes and approaches you're allowed to fly.
Full Definition
A standardized set of single-letter codes used on the ICAO flight plan form to tell air traffic control what navigation, communication, and surveillance equipment is installed and serviceable on the aircraft. The codes are entered in Item 10 of the flight plan (Item 10a for radio communication, navigation, and approach aids; Item 10b for surveillance equipment such as transponder and ADS-B), and they determine which routes, , and ATC can issue to that aircraft.