Are you interested enough in a post in your time line, that you’d like to be notified of any new activity?
To turn on notifications, click the down pointing arrow head above the post top right and select ‘Turn on notifications’. You’ll get a notification whenever someone comments on this post.
If you want to stop following the post at any time, use the same button top right and select ‘Turn off notifications’.
Note: notifications are automatically turned on for posts you write yourself or comment on (that’s why in groups you’ll often see people just add the word ‘Following’ or similar).