Grades are usually the main requirement. However, they are also looking at what you have done outside the classroom to expand the knowledge of the subject you are considering to apply to at university. Impressions from a personal statement also matter. Not to mention any useful extra-curricular activities you have done and skills you have gained from them (eg: independence, maturity, commitment, etc...). In general, they want to know if you are suitable for independent study at university and you have adequate knowledge for your course.