The construction that is shown in the accompanying diagram is the perpendicular bisector of line AB.
What is a perpendicular bisector?
A perpendicular bisector is a line or a line segment that both bisects (cuts in half) another line segment and is perpendicular (forms a right angle) to it. In other words, it is a line that passes through the midpoint of a given line segment and is perpendicular to that segment.
If you have a line segment with endpoints A and B, the perpendicular bisector is the line that passes through the midpoint, in this case (C) of AB and forms a right angle with AB. The perpendicular bisector divides the line segment into two equal parts.