The fragments plugin parses HTML responses, and looks for scripts and comments. These can be interesting, first to see if there are any hidden links, or other debugging code, and also to understand how the pages are supposed to operate. The drop down list at the top allows you to select between the two types of fragment, and the list immediately below shows unique fragments in that category that have been identified. Selecting a particular fragment in the list updates the conversation table below, showing the various conversations that included that fragment.
The Summary panel also indicates which conversations and which URL's have comments and scripts in them. If you right-click on the conversation in question, you can select a menu option that will show all the fragments of the specified type in that particular conversation.