Before the 12th Amendment, elections for president and vice-president did not require the voters to differentiate their votes between president and vice-president. It would be president the candidate who had the most votes and the vice-president would be the second most voted.
However, in the elections of 1800 presidential candidate Thomas Jefferson and vice-presidential candidate Aaron Burr ended up in a tie. That meant the election had to be decided in the House of Representatives. Jefferson won and became the third president of the US.
So in 1804 went into effect the 12th amendment that required different votes for presidential and vice-presidential candidates.
