Imagine drawing a line through a butterfly down the middle and folding it. The left wing lands exactly on the right wing. That line is the line of symmetry.
Some shapes have one line. Some have more than one. A circle has infinite — every line through the centre works.
Click anywhere on the left side. The right side mirrors you. Try drawing a face — both eyes, two ears, a smiley mouth — without moving to the right side at all.
Mirror symmetry is the fold-in-half kind — left matches right, like a butterfly.
Rotational symmetry is when you can spin a shape and it looks the same. A starfish has 5-way rotation. A square has 4. A windmill spins and still looks like a windmill.
Many things in nature have both — flowers, snowflakes, fruit cut in half.
Humans love symmetric things. Faces, buildings, flowers, music — most things we call "pretty" have symmetry hiding inside.
Scientists think we evolved to like it because symmetric things in nature are usually healthy — a symmetric flower has the right pollen, a symmetric face means the body grew evenly. Liking symmetry helped our ancestors choose good food, and good friends.