Some units are practically considered worthless by the rest of the army, either due to their low status or being simple beasts, and no heed is paid to their demise.
Models with this special rule do not cause Panic to friendly units that are not Expendable themselves. Characters may not join a unit with this rule, unless specified. Likewise, an Expendable character may not join a unit that is not Expendable.
For every Core unit with the Expendable special rule in your army, you are required to include at least one other Core Unit without the Expendable rule. For more information on this, see the Choosing Your Army chapter.