Athletes on Strava can leave comments on activities, achievements, Challenge, Race, or Goal entries and completions, etc.
Giving and Receiving Comments
From your activity feed, you can both see how many comments an activity has already received, and add your own. Tapping the Comment button from your Feed, or the Social bar from the Activity page will take you to the social display for that activity. Tapping on any comment will take you to the commenter's Profile page.
Tap the text area to bring up the keyboard, then tap Send to submit your comment. Note that if this is your own activity or comment, you can long-press on the comment from the Discussion view to remove it.
You can manage your Notifications regarding Comments from the Settings menu in the Android 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.