SpeedGrader is a Canvas tool that allows the faculty member to access, annotate, grade, and download student assignment submissions all in one place on Canvas.
Access SpeedGrader by opening the specific Assignment and clicking SpeedGrader under Relevant Items. Also under Relevant Items, there is the option to download all submissions at once, if you prefer to view and work outside of Canvas.

Inside SpeedGrader, you will see one student submission at a time and have access to document details, annotation tools, commenting, downloading the individual assignment, and the ability to quickly shift between student submissions.

For more information on SpeedGrader, please see Canvas's guide here SpeedGrader