Intercession is when someone talks to someone else on behalf of someone else.
Let's say you really want a toy that your parents say you can't have. You can ask your big brother to talk to your parents for you and try to convince them to let you have the toy. That is intercession.
In a religious context, people may ask someone they believe is closer to God to talk to God on their behalf. This person is called an intercessor. For example, if you want to pray for someone's health, you can ask a saint or a priest to pray for that person as well. You are asking them to talk to God on behalf of the person who needs it.
In summary, intercession is like having a mediator between two parties, where one person talks to another person on behalf of a third person for a specific reason, like asking for a favor or help.