What is 'mercenary spyware' attack Apple warned Indians about?

Apple warned iPhone users in India and 91 other countries about a 'mercenary spyware' attack. "Such attacks are more complex than regular cyberattacks as attackers apply exceptional resources to target specific individuals and their devices," Apple's website says. Mercenary spyware attacks cost millions of dollars and have a short shelf life, making them much harder to detect and prevent.