Before we can discuss stereoisomers, we need to understand chirality.
Chirality is the property of "handness".
Consider your hands. They are very similar structures, the fingers of each hand extend from the top edge of each palm, while the tumbs extend from the side.
Organic molecules can be chiral if they contain one or more chiral centers.
Yet your hands are not identical... they can not be superimposed on top of each other.
Look closely... your hands are mirror images of each other.
A chiral center is defined as an sp3 hybridized carbon that is bonded to four different groups.
Consider the compound 2-bromobutane: