https://www.gamespot.com/articles/destiny-2-warminds-override-frequency-guide-what-a/1100-6458839/

Among the ways in which Bungie has tried to expand Destiny 2 with the new Warmind DLC is with a dash of mystery. The Override Frequency is one such example, as they serve as a post-campaign objective for players to embark on. But the game doesn't provide a lot of information about what they are, how they work, or where to find them--here's what you need to know, as well as the potential Exotic weapon reward, Sleeper Simulant.