Mittelsenkrechte
Generally
A perpendicular bisector is a special line. It always belongs to another line which it intersects at 90 degrees. The two lines, therefore, are at right angles or perpendicular (both words mean the same). The perpendicular bisector also divides the other line into two halves of equal length.
Triangle
In a triangle, each side has its own perpendicular bisector. All three perpendicular bisectors intersect at one specific point. This point is also the centre of the circumscribed circle (which goes through all the three vertices of the triangle).