Original: Ballmer used the notion of "viral" licensing terms to express his concern over the fact that the GNU General Public License (GPL) license 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.