Plain English Meaning
It is the global rulebook that says how aviation radios, navigation aids, and satellite navigation systems must work so that aircraft can operate safely between countries.
Full Definition
ICAO Annex 10 is the international standards document covering aeronautical telecommunications, including radio navigation aids, communication systems, surveillance, and data link services used in civil aviation worldwide. It is published by the International Civil Aviation Organization and sets the technical specifications and performance requirements that member states agree to apply to navigation and communication equipment, including GPS and other satellite-based systems.