A school uniform is a standard set of clothing people (usually children) wear when they go to school. It might have a particular color of trousers or skirt, plus a matching shirt and perhaps a jacket or necktie, with matching socks and shoes.
In some countries, like Germany, students can wear anything they like when they go to school. In other countries, like England, there is usually a standard dress code in school, usually a set of dressing for girls and one for boys. Boys and girls need to wear school uniforms when they go to school. In many countries, such as the United States, some schools require wearing a uniform, and some do not.