Sensus fidelium is a big fancy term used by the Catholic Church to describe something very important. You know how sometimes you ask your parents, teachers or friends for their opinion about something? Well, that's kind of what sensus fidelium is all about.
But instead of just asking a few people, the Catholic Church wants to know what all the people who believe in God, also known as the faithful, think about a certain topic. The Church believes that when a lot of people who believe in God have the same opinion on something, then it must be a message from God himself.
This idea is based on the belief that God speaks to us through the voice of ordinary Christians, not just through priests or other Church leaders. So, it's like the Church is listening to what all the faithful people are saying and using that to figure out what God wants us to do.
Overall, the sensus fidelium is a way for the Catholic Church to make sure they are getting guidance from God through the thoughts and opinions of all the people who believe in him.