UMass Amherst has started to require two supplemental essays as part of its application, which is submitted through the Common App. Like many universities, UMass holistically reviews the applications it receives to judge the applicant on more than just GPA and test scores. This means that, while supplemental essays are certainly not the primary factor in your admissions decision, strong essays can definitely help boost your chances, especially if your application is slightly lacking in another area.
While they may change in future years, UMass has been requiring applicants to respond to the following prompts: "Please tell us why you want to attend UMass Amherst," and "Please tell us why you chose the major you did." Both supplemental essays have a maximum word count of only 100 words, so it is imperative to determine what points you want to make and form them concisely. Although they are short, they should still be treated just as seriously as other essays you submit.
After helping my younger brothers format their supplemental essays when they were applying this fall, the best advice I can offer is to try to make it unique. When answering why you'd like to attend UMass, I doubt that the admissions officers really want to read another essay about our #1 dining, even though that is certainly a compelling reason to want to attend! I also recommend being as genuine as possible and to make it personal where you can.
Good luck and happy writing!