https://www.gamespot.com/articles/destiny-2-masterworks-guide-how-to-get-masterwork-/1100-6455690/

One of Destiny 2's major new features arrived recently as part of a post-Curse of Osiris update. Masterworks present what amounts to a new class of weapon, providing players with another thing to hunt down in their journeys. Unfortunately, the way they work can be a little confusing, and as is often the case with Destiny, it's not explained well in-game. Here's what you need to know about Masterworks and Masterwork Cores.