Notes are basically a running log of the comments, reblogs, and likes a post has received. Notes are a great way to see the chronological history of the interactions of the post. The newest note will always be added to the top of the notes list, with the oldest note being at the bottom of the list. Viewing notes on a post is a great indicator of how popular the post is, and how much interest it has garnered in the community. Posts can be awesome to many and a point of debate for many. It's really interesting to read all of the notes to see what people think!
When you are looking at the notes of a post, you can click on a prior reblogger to go back to that post in the tree. Clicking on the original poster will get you all the way back to the original post. In this way, you can choose where in the tree of reblogs you want to branch your reblog from. You may be able to eliminate some reblog comments that you don't want, but never the original post.