That's how much essence is stored in the wellstone.
Wellstones and sigils automatically collect essence from plants at night in their range and share it with other sigils nearby.
you can refill your essence by approaching one

Awesome, thanks, I thought it had something to do with water flow, but this makes more sense. I was using my limited essense to build the wellstones, 13 of them during the day, running back and forth refilling it.