Ally is a tool that is embedded in Canvas that is designed to convert the format of media in your course. We refer to these as Alternative formats. These alternatives give you options to meet many kinds of needs. There are lots of reasons that you might want to convert your format. Alternative formats provide greater opportunity for everyone to access the information they need in the way they need or want it.
How do I access Ally? The alternative formats that are available are dependent on the original content. Select the Download Alternative Formats icon 
wherever you see it to download the content in a different format. Choose the version that is best for your need. While it is available anywhere in your course that you see the icon it is possible for an instructor to turn off access to some or all the formats.
How can Ally help? With alternative formats all students can meet the same learning objectives using resources that are built to target the needs of the individual student. For example, Blackboard Ally creates audio and electronic braille alternatives for students with visual impairments. It can also create audio files for students who might be working in conditions that make it difficult to read like low lighting, or on public transportation.
For more details about the possibilities and file formats watch our introductory video here (please note the video shows a Blackboard course, but it works the same in your Canvas course) - https://www.youtube.com/watch?v=2bXDH8XzMN8
For more information about the product, you can read the documentation for students at https://help.blackboard.com/Ally/Ally_for_LMS/Student