Okay kiddo, so a martyr of charity is someone who gives everything they have, even their life, to help others and make the world a better place. They do this because they love others very much and want to show that love by helping them in any way they can.
Think of it like when you share your toys with your friends or give them a hug when they're sad. A martyr of charity is doing something similar, but on a much larger scale. They might travel to far away places to help people in need, or even sacrifice their own safety or wellbeing to help someone who is hurting.
It's a pretty big and special thing to be a martyr of charity, but it just shows us how much kindness and love can make a difference in the world.