Flat shapes have sides and corners.

Look at windows, tiles and signs. They have flat shapes. A flat shape is a 2D shape.
A circle is round. A triangle has 3 sides. A square and a rectangle both have 4 sides.
Count the straight sides and the corners. A triangle has 3 sides and 3 corners. A hexagon has 6 sides and 6 corners.
A shape is still the same shape when it is turned. A square turned on its corner is still a square. Count the sides to check!
Some shapes can be folded so both halves match exactly. The fold line is a line of symmetry. A square, a circle and a butterfly all have one.