Do you know what a citation is? It's when someone mentions something that someone else wrote or discovered. It's a way to give credit to the person who originally wrote or discovered something.
Now, scientists and mathematicians write a lot of papers about their work. A mathematical citation quotient is a way to measure how often other people mention their papers.
So imagine you wrote a paper about a really cool math problem. And then a bunch of other people read your paper and use your ideas in their own work. The more people who mention your paper in their own papers, the higher your mathematical citation quotient will be. It means that your work is influential and important in the world of mathematics.