Things that have symmetry are balanced, with each side reflecting the other. A human body has such complex symmetry, from eyes, ears, and nostrils to arms, legs, and feet, that even a minor injury can make a body look unbalanced.
Snowflakes and butterflies often have a remarkable natural symmetry, with patterns on one side matched by those on the other. Objects that have identical or very similar parts lying at equal distances from a central point or line or plane — that is, objects that have symmetry — often work better. Symmetry helps boats stay upright in water. In design, symmetry is a balancing of objects, as when two candlesticks on a fireplace mantel are at equal distances from the center of the mantel. A lack of symmetry — i.e., asymmetry — might mean putting both candlesticks together at one end of the mantel.