Split ticket voting is the practice of voting for different parties in races for different offices. In the United States, we are allowed to vote for different parties in different elections. We do not simply have to vote for all of the Republicans or all of the Democrats who are running. An example of this would be a person who voted for President Obama, who is a Democrat, but who also voted for a Republican senator or representative.
The implication of split ticket voting is that people in the United States are not very loyal to one party or the other. Instead, they are more likely to vote based on the attributes of a given candidate. In many countries, people are reliable backers of one party or another. In the US, however, there is less party loyalty. People are more likely to split their tickets.
Another implication of this is that people want balance in their government. They might split their tickets because they believe that it is dangerous to let just one party have control of too many of the parts of the government.