Comments are a great way to engage with other athletes on Strava. Athletes can leave comments on activities, posts, achievements, Challenge, Race, or Goal entries and completions, etc. From your activity feed, you can see how many comments an activity has already received and add your own. To delete a comment, you will see an [X] in line with the comment when you move your cursor over it. From an activity page, you can view comments by clicking the Comments icon at the top of the page.
You can manage comment notifications from the Email Preferences sections of your Settings page. If you comment on or give kudos on an activity, you will be 'subscribed' to that activity's conversation and if your settings allow it you will receive a notification of every subsequent comment. At this time the only way to unsubscribe yourself from that conversation is to delete your comment as shown above.