Convince yourself that these images are non-superimposable mirror images by trying to line up all four groups in space. You may soon 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. Superimposing them on each other would be like lining up two right hands on top of each other.
The most familiar example of chiral objects and of enantiomers is your hands. Each hand is a chiral object, and your left hand is the enantiomer of your right hand. They are non-superimposable mirror images. Lefties are frustratingly familiar with scissors that don't work well; these are chiral objects (scissors) in a chiral environment (hands).