a molecules with single bonds can twist along that bond, a double bond cannot twist. because of this when you have group 17(7) atoms (or other groups of atoms) bonding to one of these double bonded pairs (preferbly carbon double bond) they remain static to what side they stay on.
here the clorine is bonded to the carbon in a cis formation, because they are on the same side of the molecule
Cl . Cl
C = C
H . H
this is a trans molecule because the chlorines are not on the same side
Cl . H
C = C
H . Cl
the molecules here are called cis-dicloroethene and trans-dicloroethene (i think)