The Silver Tokens was implemented on Tibia at the 2015 Winter Update, when Ferumbras' Ascendant Quest was added.
At the beginning they were used to exchange them to at the Adventurer's Guild, near of the Reward Chest, for differents types or armors. To do that you need 100 Silver Tokens.
And also they are needed to completed the Rift Warrior Addon, you can exchange 200 Silver Tokens, to obtain Addon 1 and Addon2.
Then after several updates, now with 2 Silver Tokens you can also enchant some items like: pendulet, sleep shawl, blister ring, theurgic amulet, and ring of souls. Saying: "Hi - enchant - (name of the item you want to enchant) - yes" to Cledwyn.
Hope this helps you, if you feels that I'm missing something just let me know.