Comments are a great way to engage with other athletes on Strava to provide some positive feedback - or some good natured ribbing. Athletes on Strava can leave comments on activities, achievements, Challenge, Race, or Goal entries and completions, etc.
Giving and Receiving Comments from the Activity Feed:
From your activity feed, you can both see how many comments an activity has already received, and add your own. Tapping on the Comment icon (1) from the the Activity Feed will take you straight to the Discussion page for that activity, from which you can leave a comment (2).
Giving and Receiving Comments from the Activity Page:
From an Activity page, just tap the Comments icon (3) to view and give Comments - this will again take you to the discussion page for that activity. Tap the text area to bring up the keyboard, then tap Send to submit your comment. Tapping on any comment will take you to the commenter's Profile page.
Can I delete a comment?
Yes, if the comment is your own activity or comment, you can swipe across the comment from the Discussion view to remove it:
Can I turn off notifications for an activity discussion I participated in?
You can manage your Notifications regarding Comments from the Settings menu in the iPhone app, within the Notifications section. Please note that if you comment on or give kudos to an activity, you have effectively '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.
Please note that you can now mention other athletes in activity comments. More information here.