Complex sentence: A primary (or gravitational primary) is the main physical body of a system that contributes most of the mass and will generally be located near the center of mass of a gravitationally-bound, multi-object system.
Split into simpler sentences: A primary (or gravitational primary) is the main physical body of a system. This body contributes most of the mass and will generally be located near the center of mass of a gravitationally-bound, multi-object system.