A Frog token is a reward from the frogs random event. These tokens can be exchanged for either a Prince outfit or Princess outfit (depending on gender), or a frog mask, by talking to Thessalia in Varrock, and selecting option "3. I have a frog token...". Thessalia will then prompt you with, "That entitles you to a free costume! Do you want a frog mask or a [prince/princess] outfit?"
Frog tokens cannot be withdrawn as notes and will not stack in your inventory when more than one is acquired.
Once the player has received the prince/princess outfit and Frog mask, the player can exchange tokens for additional copies of the outfit or mask.
For an exhaustive list of all known sources for this item, see here