This is the part of the application when the universities can find out more about the applicant, their enthusiasm for the course, and what would make them a good candidate to choose. Universities want to hear why the applicant has chosen that course/subject, and why the applicant considers themselves to be well-suited for the course.
It should include what interests the pupil about their chosen course, if it relates to a long-term career plan, the skills and experience they have which is relevant for the course, provide examples of the experiences and how this demonstrated relevant qualities needed for the course, and attributes that will make the pupils a good student at university.
You can read more advice for pupils on Writing Your Personal Statement.