Bungie announced that the new Solstice armour sets in Destiny 2 will have a few features that the previous ones didn't but it might sound a little confusing so here is a quick rundown.
Solstice armour sets have already featured some sort of upgrades but they previously meant the better ones would simply have higher stat totals and eventually start glowing. With Destiny 2 's Solstice 2022, you will be able to reroll stats on an armour piece but you will have to go through an entire mess of currencies to do so.
There are two main currencies needed for enrolling - Silver Leaves and Silver Ash. You earn Leaves from regular activities such as strikes, Crucible, Gambit and probably more. Once you have enough Leaves, you will need to complete the Solstice-related activity, called Bonfire Bash, which will consume the Leaves and reward you with Ash. It's this one that you will be using to reroll armour stats in the armour mod screen.
Additionally, if you want to upgrade the potential stat total on armour pieces, you will need Kindling, a third currency. This one is earned from Solstice event challenges. You can add Kindling in the armour mod screen in order to upgrade a piece of Solstice armour. Each slot (helmet, gloves, chest, legs, class item) can be upgraded up to three times but once you upgrade one, it will upgrade all the existing Solstice pieces you looted as well as the ones you will loot until the end of the event.
Essentially, it would be best if you looted an entire set of Solstice armour first and then acquired enough Kindling to upgrade all armour slots to get the best possible stat totals, at which point it would be safe to use your Silver Ash in order to reroll the stats in a way you like.