The Canvas gradebook has many features that are useful for providing feedback on assignments.
Whenever you create a new graded assignment, including graded discussions, you will get a corresponding column in your gradebook. Use the cells  in the gradebook to input grades for your assignments. All assignment grades must be entered as points, but you can change how the students see the grade in their gradebook (e.g. percentage, letter grade, etc.) or you can 'Mute' the column so students won’t see the grade at all. (You can always ‘unmute,’ or reveal, the grades to students later.)
You can provide feedback and comments on an assignment by clicking on the comment icon  in the corner of a gradebook cell. Comments are great for short notes and questions. Students can respond to your comments, and the conversation can continue for the duration of the course. The comment will appear on the Dashboard, on their “Grades” pages and in the comment stream of the SpeedGrader display for the assignment.
SpeedGrader is a Canvas tool that lets you quickly preview, comment and annotate a student’s assignment submission. Using the SpeedGrader and the DocViewer toolbar, you can mark up and add comments to the submission. Note: If you use the SpeedGrader, make sure to tell your students to click the View Feedback link when they view their submission grade in order to see all annotations and comments made on their assignment.
If the assignment being graded is a Turnitin assignment, you can access the Turnitin Originality score and report by clicking on the TurnItIn score icon  in the gradebook. Once you view the Turnitin Originality report, you can use the Turnitin Grademark tools or use the DocViewer to provide annotated comments and feedback.
More detailed information about grading in Canvas can be found in the Canvas Instructor's Guide to Grades.