It can be important for companies to know the motivation behind your application for their open position, to take it into consideration before inviting you further into the application process.
Now you have the opportunity to briefly express the reasoning behind your interest in this job. It is also an opportunity to communicate something that may not have fit into your CV.
A window will pop up when you click on apply to a position - and here you can put in the text for your motivational note.
NOTE: providing a Motivational Note is not mandatory but can give the recruiter valuable insight into why you would like to work at this company.