https://www.gamespot.com/articles/destiny-2-guide-heres-how-to-activate-heroic-publi/1100-6453219/

One of Destiny 2's many additions to existing systems is the concept of Heroic Public Events. These are harder versions of the standard Public Events you run into throughout the world and, as such, provide an opportunity to earn better loot. Unlocking them, however, is not quite as simple as pressing a button to jack up the difficulty. [Update: Curse of Osiris has been released and brings a new Public Event--and a particularly complex process for activating the Heroic. Here's how to activate Mercury's Heroic Public Event.]