To add a comment, enter edit mode by clicking Edit Mode on the toolbar or pressing Shift+E on your keyboard.
Then click on a bookmark and select Comment from the dropdown menu.
The bookmark sidebar will appear, and you can now enter your first comment.
When you post a comment, everyone on the collection will see a badge indicating the current number of comments. The badge will appear red for team members who have not yet seen a comment.
This functionality comes in handy when you're looking to have a discussion centered around a particular bookmark or want to make a note for yourself.
Here's a short video tutorial: