For two angles to be complementary means that the sum of the two angles is equivalent to 90°. When an angle has one of those little squares, that means the angle is a right angle, or a 90° angle. Also, since it's given that line segment AB and line segment BC are perpendicular, this means the two lines/line segments cross each other at a right angle, which also proves that line segment AB and BC have a 90° angle.
