The answer to this can vary, but the basic idea in a democratic system of government is that each person can only vote the same number of times that everyone else in the system can vote. In other words, there should not be people who are allowed to vote more times than others.
In most systems, each voter can only vote once in a given election. There can be many different elections (for Congress, President, state legislature, etc) on the ballot at once, but each person can only vote once in each of these races. There are systems where each person is given multiple votes and can split them up among candidates if they wish. However, the principle that each person gets the same number of votes as other persons still applies.