When two stereoisomers with a central carbon and four different substituents are mirror images of each other and non-superimposable, they are classified as enantiomers. This is equivalent to the right-left hand relationships: your hands look identical, but if you try to pile one on top of the other, you’ll see that’s impossible.
The concept of stereoisomers is very close to the one of chirality and the two can be easily confused. A chiral molecule has a carbon with four different substituents that have lost any kind of symmetry, known as asymmetric carbon. Therefore, two chiral molecules can be distinguished only by their absolute configuration in space. If a molecule has two or more substituents, of the same nature, therefore a plane of symmetry, the molecule is considered achiral.
Remember that stereocenter and chiral center are often used with the same meaning, but there is a slight difference: while stereocenters don’t necessarily imply four different substituents, chiral centers do. All chiral centers are stereocenters as enantiomers are a subgroup of stereoisomers, but the vice-versa is not always true, as diastereomers also have stereocenters.