Adobe Reader is free software provided by Adobe that allows users to open PDF documents. In most cases, it is not necessary to install Adobe Reader to view PDF documents due to PDF viewer functionality being "built-in" to so many of the devices and software that we use on a daily basis. For example, all major web browsers include a PDF viewer, Windows and Apple computer operating systems have built-in PDF viewers, and even Brightspace is able to render PDF files that instructors upload to Content, so they can be viewed directly from within the Content tool.
In some cases however, you may need to fill out or digitally sign a form that is downloaded from the Internet and that may mean the document needs to be opened in Adobe Reader. If you run into this, you can install Adobe Reader for free by visiting https://www.adobe.com and clicking on "Adobe Acrobat Reader", which is typically located near the very bottom of the page.