Confused....Still

ok so i've had tumblr for like a couple of months now and i still haven't really got the hang of it.

so you know how you reblog and then comment on it,so you link to your blog then put your comment right?well everytime i do that i see that everyone else has this grayline going down right below their tumblr name...what is that or how do you do that because everytime i comment that grayline doesn't appear.. [example]

tumblrname:
[
[ < (grayline)
[
[
[lol omg wtf hahahaha forever reblog.
[
[