A Letter of Motivation is a letter which accompanies your application and acts as a means of introducing yourself to the programme coordinator and the admissions committee.
Your letter should contain specific information about why you think that you should be given a place in the programme. You should detail your skills and explain the reasons why you are a good fit for the programme. Also, your letter should serve to convince the reader that you are sufficiently interested in the programme to merit further attention and a very careful screening of your complete application package.