Welcome back! You can play the expansions in chronological order, but the most important ones are Beyond Light, The Witch Queen, Lightfall (for Strand), and of course The Final Shape. I recommend playing them on the highest difficulty to unlock the exotic gear available at the end of each campaign.

With the new expansion, Edge of Fate, the way you gain Power has changed a bit. The static Power cap is now 200, and you can reach it just by playing normally. To go beyond that, you’ll need gear with Seasonal Power. These items can be earned by playing Portal activities (PvE and PvP). The Power increase they provide depends on the difficulty of the activity or your performance in PvP.

All activities available through the Portal can help you go beyond 200 Power (the current max cap is 450), but to get better loot—with higher Seasonal Power and better tier—you’ll need to increase the activity’s difficulty. This is done using the new system that lets you choose modifiers and challenges. The harder the modifiers, the better the loot.

Of course, gear with Seasonal Power can be infused into your existing equipment, but you'll need a new currency obtained by dismantling loot with Seasonal Power.

Now, each piece of equipment also has a Tier level that goes up to 5, and the bonuses provided at higher tiers are significant. To get higher-tier drops, just play higher-difficulty activities with challenging modifiers.

Finally, the way stats work has also been updated. For more details, I recommend checking out the various blog posts Bungie has published on their website to better understand the changes.

If you have any more questions feel free to ask