Perpendicular bisector of a line segment

The perpendicular bisector of a line segment is a line that passes through the midpoint of the segment (it bisects it, or cuts it into two equal pieces) and it makes a  angle with it (is perpendicular to it). In the sketch below the red line pb is the perpendicular bisector of the line segment AB. A and B can be moved to change the segment and D is a point on the perpendicular bisector. Draw the segments joining D to A and B.

How are the distance from a point on the perpendicular bisector to the endpoints of a segment (in our case DA and DB) related? What does that mean?