Sometimes teachers want to work on grades without students seeing the grades until a later date. (For example, if a TA is grading, but the teacher wants to double-check and possibly adjust the grades before students see them, or if grades may change for some other reason.)
Canvas has variously called this "hiding grades" or "muting grades" in the past, but their current terminology is about a "grade posting policy" being either automatic (grades posted right away) or manual, where grades aren't posted until you specifically post them. By default, grades are posted automatically unless you change that column to be manual. In the Gradebook, the word "MANUAL" will appear in the column heading if that column has been set to manual.
To hide grades while you're grading, do this: Go to the Gradebook, choose the menu at the top of the column (those three dots) and choose Grade Posting Policy and choose Manually.
You can then grade those assignments (by typing in grades or using the SpeedGrader) and students will not see any of the grades.
Next, when you're done grading and ready for students to see the grades, chose the menu, then "Post Grades"Â
Note the little cross-out eye symbol in the top-left of the column header. That means that there are grades in that column that have not yet been posted.
For more details on this change, see the full Canvas documentation at: How do I use posting policies in a course?.