Bacha posh is when families in some countries - like Afghanistan - dress up their daughters as boys. This means they wear boy's clothes and are called boys too. Even though the girls are still girls, people treat them like they are boys now.
This might seem strange, but there's a reason behind it. In Afghanistan and some other places, boys are seen as more valuable than girls. Boys get to go to school, get better jobs and have more freedoms. Families may also need a boy to help with work or protect the family.
So some families pretend that their daughters are boys. This way, they can enjoy the benefits and freedoms that come with being a boy. The girls with their new boy names can go to school, run errands or help out in the family business.
Sometimes, these girls go back to being girls when they grow older or get married, while other times, they may never go back. Bacha posh is not always allowed or accepted by everyone, but it's a way some people have found to give their daughters a better life.