Popular vote is the vote of the citizens. It is basically the direct vote that the candidate receives from the masses. In the U.S however, the Electoral College (made up of 538 electors) is what is used to elect Presidents. So it is possible to win the popular vote, and still lose the presidential election (If the person does not win at the Electoral College).
However, when the masses cast popular vote, the vote goes to the electoral college, and most times, the members of the electoral college reflect the popular vote in their state.