This token is placed on an Action space to remind you that the next Success result rolled while performing that Action must be rerolled. After this has happened, discard the reroll token.
Note that, unlike other special tokens and markers placed on Action spaces, the reroll token remains on the space until a Success has been rerolled. If you perform the Action on that space but don’t roll a Success, then you don’t have to reroll and the marker remains in place.
Some cards (Ceremonial Bowl, Confused) instruct you to place a reroll token on your character sheet. In this case, the effects of this token are described on the cards.