To comment on an issue, first click on the issue to open it in the left side panel. Scroll down to the bottom of the Issue to the “Comments” section. Type your comment in the ‘Add a comment…’ field and hit ‘enter' on your keyboard.
When a new comment has been made on an issue thread, the owner, assignee, and any other user who has commented, will receive a notification. The notification can be viewed by clicking on the bell icon in the bottom corner of the left sidebar.