Теория:

A line which meets another line at right angles is called perpendicular line. A bisector cuts a line into two equal parts. Therefore perpendicular bisector is a line which cuts another line segment into two equal parts at $$90$$ degrees (right angle).
Construction of a perpendicular bisector line:

Step 1:  Draw a line segment PQ.

Step 2: Using  a compass, with P as a centre and radius more than half of the length of line segment PQ, draw two arcs of the same length one above the line PQ and one below PQ.

Step 3: With the same radius, Q as centre draw two arcs to cut the arcs drawn in step 2. Mark the point of intersection of arcs as R and S.

Step 4: Join the line R and S, name the point of intersection as O. This point is called midpoint of the line PQRS is the required perpendicular bisector of line PQ.