Every chiral centre in a molecule has two possible ways of putting the groups in space. With only one chiral centre, there are two enantiomers. With two chiral centres, you can form 4 different isomers. When all chiral centres are changed at the same time, two pairs of enantiomers are formed. When only one chiral centre is changed at a time, sets of diastereoisomers are formed. With more chiral centres the possibilities are many more.