Clicking each of the buckets will show you various applicants who fit the description. An applicant can be in more than one status (for example, Favorites and Pipeline). When you click a bucket and then want to go back to view the rest of your applicants, click Reset All Filters.
Here's what clicking the various filters will show you:
Clicking Favorites will show you applicants who you have marked as a favorite. More info: What does adding an applicant as a "Favorite" do?
Clicking Pipeline will show you applicants who you have moved to the Pipeline to be considered for other positions. More info: What does moving an applicant to the pipeline do?
Clicking Hired will show you applicants who you have hired from CareerPlug.
Clicking Needs Review will show you applicants on whom you have not yet taken any action. Clicking this filter is a good way of viewing your new applicants.
Clicking Disqualified will show you applicants who gave a disqualifying response to one of the prescreen questions on the job to which they applied. More info: How do I use prescreen questions?