At base, every 8 seconds while in combat you get a stack of mists/clouds. Max is 10 stacks. When you use the Sheilun spell, the stacks are consumed and you heal up to 3 nearby allies for 8835 per stack. So at 10 stacks, 8835*10 = 88,350 healing on up to 3 nearby allies.
If you pick the talent that reduces it to a cloud every 4 seconds, you’re doubling the rate of growth and therefore able to cast the spell more often.
If you pick the OTHER talent that grants you a buff, it works like this: Every stack of clouds grants you 3 seconds of a random buff. At 10 stacks, you get one of the 4 random buffs for 30 seconds.
I had to play around with it to kind of get a grip with it. It seems like the talent that reduces it to a cloud every 4 seconds would be best in dungeon content, while the other one that gives you a big stat buff would be best in raid content where those buffs can be applied to your other spells that do much more healing.
Hope that helps. I also hope I didn’t mess something up, haha