Whenever you get a weapon that is a higher level than you can equip you can use a rift stone. For example: I am level 70. I bought a sword that's level 90. 2 options either level up to 90 to use it or use a rift stone. So I open the equipment screen try to equip the sword it says as an option "release equip level" So I use a rift stone level 90. Viola I can now equip it. I can unequip it and the effect remains. If I trade to another character the effect disappears. The level of the rift stone can unlock equipment up to that level. So a rift 90 can unlock levels 1-90 A rift 30 can unlock 1-30. If you use your rift stone 90 I would recommend making sure the equipment you use it on is 5*+.