When you're working with others on a shared presentation, everyone with editing permissions will be able to make changes. When someone else is working in the same presentation as you, you'll see what part of the presentation they're working on, and their changes will appear as they make them. Changes you make will likewise be visible to everyone else editing the presentation.
When someone else adds a comment, you can reply to it to give your own feedback. A slide with a comment will be marked by a comment icon in the slide navigation pane.