Unvoting is the action in which one cancels the vote they have made. For example, say Bob voted Julie, but changes his mind. Bob cancelling his vote is to unvote. It is similar to retracting, as both involve undoing an action, whether it be a vote or a role-claim.
There are many uses for unvoting. Mainly, when someone claims a role to stop the town from lynching them, the town will proceed to unvote if they believe the one claiming is telling the truth, or at the very least to decide. As well, unvoting can be used to avoid a blitz and reset the timer.