What is steric strain in organic chemistry?

Steric strain is the increase in potential energy of a molecule that is caused by van der Waals repulsions and cannot be reduced by rotation around a single bond.

We often find steric strain in alkenes and ring systems.

Alkenes

van der Waals repulsions between the eclipsed chlorine atoms cannot be reduced by rotation around the rigid carbon-carbon bond.

This is why cis alkenes are less stable than trans alkenes.

Ring systems

Groups in the axial positions of cyclohexane experience van der Waals repulsions from the other axial groups on the same side of the ring.

No such problems exist for equatorial substituents

This is why equatorial positions are more stable than axial ones.