Okay kiddo, so when someone is baptized in the Mormon church, it means they are making a promise to follow God and Jesus Christ. They also become a member of the church.
When someone wants to be baptized in the Mormon church, they usually meet with the church leaders to make sure they understand what they are committing to. Then they choose someone to perform the baptism, usually a priest or someone who has the authority to do it.
The baptism itself happens in a body of water, like a pool or a river. The person getting baptized will go into the water, usually wearing white clothes, and the person performing the baptism will say some special words and then dunk the person under the water for a moment.
This is supposed to symbolize washing away any sins and starting a new life following God. Afterwards, the newly baptized person is welcomed into the church community and is given a special blessing.
So, that's basically what baptism in the Mormon church is all about- making a promise to follow God and becoming a member of the church through a symbolically cleansing experience.