A line of symmetry divides a shape into two matching parts, which will overlap each other exactly when folded along the line of symmetry.
When we complete shapes to make them symmetrical, we must reflect each corner in the line of symmetry.
Look at this shape.
To complete the symmetry of the shape, we reflect each corner in the line m.
Now look at this shape.
Notice that there is a gap between the line of symmetry and the shape.
When we reflect the shape, we make sure that there is the same gap on the other side of line m.