just wondering what does it mean when it says"Notify me when a reply is posted". I always just erase the check mark beside it. If I keep the check mark, what will happen?

Tree Dude

It means that when someone has replied to a post that, either you orginally posted, or you replied to, you will receive an email letting you know that there has been an additional post attached to the orginal post.
Usually an answer or sometimes asking additional questions.
It just lets you know there has been additional activity to the orginal post
Try it, think you will like it.

Remember, you can always go back and uncheck it if you do not like this feature. Personally, I find it extremely useful. I don't have time to read each and every post. This feature keeps me updated on replies to things I have posted and posts to which I have replied.