Answer:
In a presidential election, the popular vote simply means an aggregate of all voters from all states in America. The candidate who gets the most votes nationwide is said to have won the popular vote. But the winner of the popular vote may end up losing the election, like Al Gore did in 2000 and Hillary Clinton in 2016. In the 2016 presidential election, Hillary Clinton won 48.2% of the popular vote compared to Trump's 46.1% but lost the electoral vote by a substantial margin (304 to 227). In 2012, Mitt Romney won 48% of the popular vote but only 38% of the electoral vote.
This discrepancy occurs because although Americans vote directly for their chosen candidate in the presidential election every 4 years, the president is elected by the institution called the Electoral College. This article explains the difference between the electoral vote and the popular vote, i.e., how the Electoral College system works.
Comparison chart
Electoral Vote versus Popular Vote comparison chart
Electoral Vote Popular Vote
Political Structure Representative republic Direct democracy
Progression of Vote Citizen votes for delegate or representative, generally in accordance with their allegiances/party affiliation. Delegates convene and vote. Winner of that vote is elected for the position in question. Citizens vote for their choice of official for the position being elected. Votes are counted. Majority of votes is elected to that position.
Bureaucracy Requires formation of some form of committee, college, or council to vote after they've been elected. May also have government oversight organizations. Requires no formation of such groups, nor the election of such groups. May also have government oversight organizations.
Establishment of Voting Districts Mandatory, regional delegates run for given district's delegate locations via their party or individually. Not required.
Gerrymandering Present and created as a result of voting districts. Not created due to lack of need for voting districts.
Party Benefits Favors majority parties, as they can concentrate resources, change bureaucracy, establish and gerrymander voting districts. Favors no party size in particular, though greatly improves potential for minority parties e.g., a third political party in the U.S.
Modern History Does not allow higher populated areas (say, CA or NY) to take advantage of being able to always vote for the candidate, thereby underrepresenting the other rural areas of the nation. Thus, it safeguards smaller state interests. Harder to accomplish beyond geographically-close groups prior to modern transportation and communication. These hindrances are no longer in place for developed nations