Vote-splitting is what happens when two or more people running for the same position are so similar that they draw votes away from each other. Imagine it’s time for everyone to pick their favorite ice cream flavor. If three friends all love chocolate ice cream, but they each make their own ice cream flavor, they might all get some of the chocolate lovers' votes. This could make it harder for any one of them to win over someone who likes a different flavor.
Similarly, in elections, if two or more candidates have similar ideas and policies, they could end up getting votes from the same group of people, making it harder for any one of them to win against a candidate who has different ideas and policies. This is why sometimes people say that a vote for a particular candidate could actually help another candidate win if the vote is split between similar candidates.
So, to sum up, vote-splitting means when two or more candidates are very similar and they end up competing for the same votes, which could make it harder for any one of them to win the election.