Hi @david_newman1 ...
According to these Guides, the dash means "No submission".
I know this is only a partial answer to your question, but I'm hoping someone may be able to help answer the second part of your question ... as I am administrator for our Canvas environment and don't teach courses.
Good luck. Stay safe, and be well.
Hi David Newman - since you see a checkbox that says "Calculate based only graded assignments", you are looking at one student's grades rather than the grading grid, the complete gradebook. Students use that for their own information only, and it doesn't affect their actual grade, just their view of it. Students use that after doing only a few assignments in a course, before the rest are due.
If you look at the complete gradebook in grid view you don't see that checkbox. If the student was required to submit an assignment but didn't, then leaving that dash " - " means it doesn't affect his gradebook total, it won't be counted. You should enter a 0 as score for missed assignments to make the total accurate. Imagine a student submits only one of 20 required assignments for the entire course, but got full points for it. The gradebook total will show 100%. If you enter 0's for the other 19 items the total will be more accurate.