Simplify the following text for a Grade 13 native English-speaking person.
Original: Ballmer used the notion of `` viral '' licensing terms to express his concern over the fact that the GNU General Public License ( GPL ) employed by such software requires that all derivative software be under the GPL or a compatible license .
Simplified: Ballmer used the term `` viral '' licensing terms to show his worry that the GNU General Public License ( GPL ) license forced by such software requires that all software made from it be under the GPL or a compatible license .