If you don't know what /r/place is, it's essentially a unique Reddit board in which any user can place a single-pixel anywhere on the screen.
As explained in the latest Bungie blog post, the devs have cooked up this new free-to-claim cosmetic after two Destiny 2 communities based on Reddit came together to illustrate the looter shooter on the famed /r/place board. Bungie always tries to reward the Destiny 2 community with free cosmetics and events to keep it engaged with the action MMO, and the newest Crushed Gamma emblem is the latest example.