Median Voter Theory is a way to think about how people will vote in an election. It says that people's votes will be mostly swayed by the opinion of the person in the middle of the group, which is called the "median voter". The median voter is the one who has slightly more people with opinions on either side. For example, if you have situations with 100 voters and their opinions, the median voter would be the 50th person.
To understand it another way, if you have a group of friends, the median voter in this group is the one who usually decides what everyone will do. The other voters in the group usually care about what the median voter thinks, so their opinions will end up swaying a lot of the decisions in the group.
In an election, the same thing usually happens. The median voter's opinion is important, so their vote usually decides which candidate will win the election.