In organic chemistry configuration means time-independent relative disposition of atoms and groups in space within a molecular species.
Compounds 1 and 2 have the same atoms: two carbon atoms, two hydrogen atoms, and two chlorine atoms. They have the same connectivity: each carbon atom is doublly bonded to the other and bears a hydrogen atom and a chlorine atom. However, in 1 the two hydrogen atoms are on the same side of the double bond; so are the two chlorine atoms. In 2 the two hydrogen atoms are on the opposite sides of the double bond; so are the two chlorine atoms. Since there is no free rotation around a double bond, the above relationships of atoms in 1 and 2 do not change with time. In other words, 1 and 2 can not be interconverted by rotation around the double bond. Compounds 1 and 2 are said to have different configurations.
see also conformation