The fragment browser is the panel that lists all fragments in an animation database (and its sub-databases). It is used to find & create fragments, change fragment tags, as well as creating and editing fragmentIDs.
It is typically used in combination with either the fragment editor (to edit fragments) or the previewer (to preview sequences).
It can be accessed by clicking on the "Fragments" tab at the left side of the editor.
For tutorials that go through some of this functionality, check out Mannequin Editor Tutorial 1 - Preview Setup, Fragments and Saving and Mannequin Editor Tutorial 2 - Tags & Previewing.
The next sections will go through the parts of this panel from top to bottom:
Use the context dropdown to select which context data you would like to preview. This list maps to the list inside the current Preview Setup, which can be edited in the Context Editor.
The context data refers to the animation database whose fragments are displayed in the browser.
Shows the ADB in which fragments for the currently selected fragmentID end up in.
This ADB is not necessarily the same as the ADB the currently selected fragment ends up in. This function only checks the fragmentID, not the tags.
Shows the list of fragments, sorted by fragmentID.
The currently 'edited' fragment is shown in boldface. This is the fragment whose tagstate is displayed below, and it is the same fragment which is the main fragment currently displayed in the Mannequin Fragment Editor.
Note that this is a different fragment than the currently selected one! (see screenshot above).
This shows the tagstate (both global tags and fragtags) of the currently edited fragment, which is the boldface fragment in the fragment list.
Here you can edit this fragment's tagstate. An empty or unchecked tag means the tag is not set on the fragment.