The video introduces the concept of symmetry using a butterfly image, explaining that if two sides of an object are mirror images, they are symmetrical, and the dividing line is called a line of symmetry.

It then demonstrates lines of symmetry in basic shapes, starting with an isosceles triangle, which has one line of symmetry running down the middle.

The video moves on to a rectangle, explaining that it has two lines of symmetry: one vertical and one horizontal, because each half reflects the other across these lines.

A tip is provided: to check for symmetry, draw a line and observe if the two resulting sides appear symmetrical.

Finally, an equilateral triangle is used as an example, showing that it possesses three lines of symmetry, each extending from a corner to the midpoint of the opposite side.