You can make a soulstone by having a soulshard in your inventory and casting the ability "Conjure soulstone".

An item called "soulstone" will then appear in your inventory replacing the soulshard you used. If you right click it while targeting someone (yourself or another party member) they gain a buff called "Soulstone ressurection"

Normally when you die, theres a box that says "Release spirit". But if you die with the "Soulstone ressurection" buff on you, then the box that normally says release spirit also says "Soulstone ressurection". If you click on Soulstone ressurection then you revive instantly where you died with the amount of health and mana that the ability said you would return with.

Soulstones are items that Warlocks conjure that when applied to a group member or yourself allow that party member or yourself to resurrect after death. To actually use a soulstone you have to first conjure one and then apply it to a group member or yourself by having the individual targeted and clicking on the soulstone. Keep in mind also that soulstones have a 30 minute cooldown.