Convince yourself that the pair of structures on the left are non-superimposable mirror images by trying to line up all the groups in space. You will recognize that you can line up the molecules so that they appear to be mirror image reflections of each other, like your hands are mirror images of each other. Remember that non-superimposable mirror images are called enantiomers. Superimposing them on each other would be like lining up two right hands on top of each other.
The pair of structures on the right are also enantiomers. If you compare one structure from the left pair and one from the right pair, you will find that they are also non-superimposable. You should also recognize that they are not mirror images of each other. These types of stereoisomers (non-superimposable, non-mirror images) are called diastereomers.