The segment addition postulate states that if we are given two points on a line segment, A and C, a third point B lies on the line segment AC if and only if the distances between the points meet the requirements of the equation AB + BC = AC. See Diagram 1 to gain a clearer understanding of this postulate definition.
Remember that a line segment is part of a line bound by two clear end points. It is comprised of a bunch of points between those two end points.
An easier way of stating the segment addition postulate is that if point B lies on line segment AC, then AB + BC will equal AC. That seems pretty simple, does it not?