Each post on the blog has a list of "labels" at the bottom. These labels coincide with the subject matter of the post.
On the right side of the blog, there is a section called "Labels". This section is a list of all of the labels which exist on the entire blog.
You use these labels if you would like to see only those posts relating to a certain subject. For example, if you click on the "parenting" label, the blog will show only the posts that are about parenting.
The number after each label is the number of posts you will find listed under that label.
This makes it easy for you to quickly view a list of topics that have been discussed on the P.E.T. blog, and then bring up posts under a category you're interested in.