This is a very good way to show a comment at the top. Imagine you find a very funny or important comment and you want show it to someone, when you send the video url and tell the other person to find the comment, then that person might not find the comment at all. But if you do this then he will get the exact same comment that you want him to read.
Step1: Got to the comment, doesn't matter if it's a sub-comment or the main comment. (0:17)
Step2: Highlight the comment (you don't need to highlight all of it, you can just highlight the beginning part) with the commenter's name (0:23) and the right click on it.
Step 3: After clicking on the "View Selection Source" copy the comment Id.make sure you copy it including "lc=".
Step4: Now got to the address bar and paste it right after the video id, but make sure you add "&" after the video Id and hit enter. For example https://www.youtube.com/watch?v=QXfhGxZFcVE&lc=z135in1qdn2ncnd2r22jfrng4keofrpbu04 So the yellow highlighted text is the video id and green highlighted text is comment id, but things in between them are important too.
Now you can just copy the hole url and send it to who ever you want and that person will find the exact same comment at top of all comments.