If you're new to Shadowkeep, I can gaurentee you've seen some red glowing orbs drop from enemies, seen that it gives you a buff, and you haven't a single clue what it is, or what it does. Unfortunately the game doesn't tell you (pretty standard for Destiny 2). Thankfully, Unstable Essence's are pretty straight forward:

1. Any "Lesser Nightmare" creature will drop an Unstable Essence when it's killed. 

2. Unstable Essence's stack in duration, not in potency.

3. Unstable Essence provides a damage buff that ranges from 1.8% to an 80% increase. As the name suggests, it's unstable, so a huge boost to your damage output isn't gauranteed.

4. Unstable Essence only buffs your damage against other Nightmares. As you've probably noticed, these enemies are much tougher than their peers, resulting in you relying on Unstable Essence to reliably (and safely) cut through them. 

5. The remaining duration of Unstable Essence is displayed on the left-hand side of your HUD.