Among the many changes that came to Destiny 2 with Beyond Light is the introduction of a new way to get old Exotics that you might have missed out on in previous seasons and DLCs. To facilitate this, the game also has a new currency, the Spoils of Conquest.
For many of the Exotics, which can be found in the Archive in the Tower, you will need Spoils of Conquest to get them, along with an Exotic Cipher, and some other resources to boot.
To get Spoils of Conquest, you need to take part in Raids. There are only two Raids in the game at the moment, the Last Wish and the Garden of Salvation. The Deep Stone Crypt will be added to the game on November 21. When you finish the raids you will be rewarded with some Spoils of Conquest.
How you spent them is up to you, and will mostly depend on what Exotics you are missing from your collection. It is a good way to get old, powerful weapons that will be relevant forever because Exotics do not have to fear things like Sunsetting.
The only downside is that you will need a team of six people to run these Raids, so if you are not able to get a group together, you might need to look to an LFG service, or perhaps hope that Bungie have another way to get the Spoils of Conquest hidden away in the game that we have not discovered yet.