Each voter may vote for (approve of) as many of the candidates as they wish. Each voter may vote for any combination of candidates and may give each candidate at most one vote.
How the Results are Calculated
A simple vote counting algorithm is used to count the votes cast on the ballots, and the winner is the candidate (or candidates, in the event of a tie) who receives the most votes (i.e. is most-approved).
For further details visit Approval Voting on Wikipedia