What Is a Provisional Ballot ?

Since Broward County Election offices and trucks seem to have so many boxes of "Provisional Ballots" just laying around to be counted as needed, this is what they are:

In the US, a Provisional Ballot is used to record a vote when there are questions about a given voter's eligibility that must be resolved before the vote can count.

Sauce: http://www.ncsl.org/research/elections-and-campaigns/provisional-ballots.aspx