Illustration of enantiomers
In chemistry, isomers are different molecules where the relative number of atoms is the same, but the arrangement of those atoms differs so that they are not identical. Enantiomers are a subtle type of isomers where the molecules are non-superimposable mirror images. The graphic illustrates this for a generic amino acid. After reflection in a plane of symmetry and rotating the result 180 degrees it can be seen you end up with a different molecular structure because they cannot be superimposed.