Martyrdom in Christianity means that someone is willing to die for their beliefs in God and Jesus.
Imagine you have a favorite toy that you love very much. You take care of it, play with it all the time, and even sleep with it every night because you love it so much. Now imagine someone comes and tries to take your toy away from you by force.
If you refused to give up your toy and you were hurt or killed because of it, people might call you a martyr because you were willing to suffer for what you believed in.
The same thing happens in Christianity. Martyrs are people who refuse to give up their belief in God and Jesus, even if it means they might be hurt or killed for it. They are willing to suffer and even die because they believe their faith in God is more important than anything else.
Many martyrs in Christianity are remembered because they were killed for their beliefs, and people believe they went to heaven for their sacrifice.