Complex sentence: In this special inertial frame where the center of mass it at rest, the total linear momentum of the system is zero, and the total energy of the system is the minimal energy as seen from all possible inertial reference frames.
Split into simpler sentences: In this special inertial frame where the center of mass it at rest, the total linear momentum of the system is zero. Also, the total energy of the system is the "minimal energy" as seen from all possible inertial reference frames.