Plain English Meaning
Galileo is Europe's version of GPS. It's a network of satellites that tells receivers on the ground exactly where they are. Aircraft equipped with the right receivers can use Galileo by itself or alongside GPS for better, more reliable position information.
Full Definition
Galileo is the European Union's global satellite navigation system, providing positioning, navigation, and timing services through a constellation of satellites in medium Earth orbit. It is one of several Global Navigation Satellite Systems (GNSS), alongside the United States' GPS, Russia's GLONASS, and China's BeiDou. Galileo signals can be used independently or in combination with other GNSS constellations by suitably equipped receivers to improve position accuracy, availability, and integrity.