I'm not sure if you are students or instructors, but I will provide you with some resources for both roles:
After reviewing these two video tutorials, if you want to take a deeper dive as to how discussion topics work in Canvas, I would recommend that you use the links that Stephanie provided in her above response...as those are the written Guides all about discussion topics.
I hope this helps! Sing out if you have any questions. Take care...stay safe...be well.