Cis–trans isomerism is a type of isomerism, which is when two chemical compounds are made of the same amounts of the same elements but the way the atoms are connected is different. This type of isomerism happens in chemicals that can't rotate around an atom or chemical bond, keeping different parts of a molecule "locked" on the same or opposite "sides" of a plane containing that bond or atom. If the important parts of a molecule are on the same "side", it is called the cis configuration, and if they are on opposite "sides", it is called the trans configuration.
This kind of "lock" is common when molecules have a flat molecular geometry. A common type of molecule with these kinds of isomers have double bonds between carbon atoms, or more rarely other tetravalent elements like silicon. All six of these atoms - the two in the double bond and their four substituents - are in a single geometric plane, and are locked on different sides of the perpendicular plane containing the bond.
Coordination complexes with planar geometry also show this kind of isomerism: cisplatin and transplatin are the cis and trans isomers of PtCl2(NH3)2, but only cisplatin works as an anticancer drug. Whether a ligand is cis or trans to another ligand can change how the complex reacts; this is called the cis effect or trans effect.
Another type of molecule that shows cis-trans isomerism are cyclic compounds. Cyclohexane is not a planar molecule, but the ring of atoms means one atom can't rotate without also moving the rest of the ring (a ring flip). This means that while substituents can cross the plane through the ring, they can only do so along with every other substituent on the same side, so they stay cis or trans relative to each other.