Sectarian violence among Muslims happens when people who believe in different things within the same religion hurt each other. It's like when children argue over toys and it turns into a fight. Muslims have different groups in their religion, like Sunni and Shia, who don't always agree on everything, and sometimes this can lead to fighting and hurting each other.
Imagine that you and your friend believe in different ways of playing a game, but you both love playing together. One day, one of you starts to say that your friend is playing the game wrong and starts hurting him/her. That's what happens when sectarian violence among Muslims occurs. One group of Muslims thinks that the other group is doing something wrong, and so they start hurting each other.
This violence can be really bad and even deadly, and it often causes a lot of sadness and pain for people in the Muslim community. It's important for Muslims to remember that they are all part of one religion and to work together to find solutions to their disagreements without hurting each other.