A PyMOL Fragment is a special concept which relates to picked atoms in the molecular editing mode.
When atoms in a molecular are picked, a striped globe will appear around every picked atom. These atoms can be thought of as joints which divide the molecule up into a set of fragments. For example, in the figure below, three picked atoms divide the molecule up into four discrete fragments.
Because various actions, such as coloring, rotation, and removal can be selectively applied to fragments, picking of atoms to define fragment can provide an efficient way of modifying and manipulating connected subsets of a molecular graph.