The drawings you made are constructions of the perpendicular bisector. The "perpendicular" refers to the fact the second line meets the first line at a right angle (aka 90 degree angle). The "bisector" portion indicates the segment has been bisected, which is mathematical way of saying "cut in half".
The key here is the "bisector" portion. Because we've cut a segment like AB in half, this means segment AC and CB are the same length. The point C is the midpoint of AB. Point C is formed by intersecting the perpendicular bisector and the original segment. I'm referring to drawing (a), but the same idea applies to drawing (b) as well. I recommend using another letter than C for the second drawing.
