Okay kiddo, let me explain what a "votebank" means.
Imagine that your class is going on a picnic and the teacher asks everyone where they want to go. Some kids like beach, some want to go to an amusement park, some want to watch a movie, and so on. But there are a big group of kids who want to go to the zoo.
Now, if the teacher was really smart, she might realize that if she wants the majority of the class to be happy, she should choose the zoo. The teacher just gained a "votebank" of students who want to go to the zoo.
In the same way, politicians might try to win the support of a particular group of people who all share similar interests and opinions. They might try to make promises to this group to gain their votes, creating a "votebank" that they can rely on in elections.
For example, a politician might promise to improve healthcare for older people, knowing that older people make up a big "votebank" that can help them get elected.
So, "votebank" is basically a term used to describe a group of people who share similar interests and opinions that politicians try to win over by making promises or taking actions that they think will gain their support.