Final answer:
The concept in question is the geometric property of a midpoint dividing a line segment into two congruent segments.
Step-by-step explanation:
The student's question involves the concept of midpoints in geometry. According to the provided information, if we have a line segment AC, and point B is the midpoint, then B divides AC into two congruent segments, AB and BC. This concept is central to understanding properties of line segments and their divisions in geometric figures.
When a point is the midpoint of a segment, the two halves are equal in length, and thus are congruent segments. This is a foundational concept in geometry that often applies to further problems involving triangles, angles, and other geometric shapes.