Marriage equality is when everyone has the same right to marry the person they love and want to spend the rest of their life with. It doesn't matter if you are a boy or a girl, if you love someone of the same or opposite gender, or if you believe in a different religion or have a different skin color.
Before, some people weren't allowed to marry the person they loved just because they were different from what some people thought was "normal." This was not fair and hurt a lot of people's feelings. People fought for many years to change the rules so that everyone could have the same rights and be treated equally.
Now, people can marry whoever they want if they both agree and are of legal age. This means that everyone - no matter who they love - can share their life and love with another person and be accepted by the law and society.