Very often schools ask specific questions that you need to answer in writing in addition to your references, cover letter, and resume. These essay questions are usually the most important part of the application as they stress what the school considers important: creativity, analytical skills, structuring skills, achievements, ambitions etc.
However, it is really important that you pay close attention to the instructions and the questions themselves. If a remark reads "please answer the questions in no more than 500 words" then do not use more even if you want to and think it is necessary. It is not! Some schools might reject your application simply because of not being able to pay attention to detail or following simple instructions. And very often these remarks are given to protect the reviewers from too much work as they might be drowning in applications. Keep in mind that your application is not the only one a reviewer has to read. Reviewers will most likely be swamped with a deluge of applications. Non-compliance to such simple requests may very well be used as a criterion to sort out a first batch of applications.
Example questions might be: