What is a perpendicular bisector?

A perpendicular is a line at 90°.
And a bisector is a line which divides a line segment (or an angle, called angle bisector then) in two equal halves.

So a perpendicular bisector is a line at 90° which divides a line segment in two equal halves.

Example: Suppose an isosceles triangle ABC where angle B = angle C, then a line from point A perpendicular to line segment BC will be the perpendicular bisector for the line segment BC.

Note: It will also be an angle bisector as it will divide angle A in two equal halves.