Steric strain is the increase in potential energy of a molecule due to repulsion between electrons in atoms that are not directly bonded to each other.

eg:  Consider two staggered conformations of 1,2-dibromoethane:

In 1, the bromine atoms are closer to each other than they are in 2.  (in 1, the dihedral angle between the bromine atoms is 60º; in 2, it is 180º.)  Thus, the repulsion between electrons in one bromine atom and those in the other is greater in 1 than in 2.  Thus, steric strain is greater in 1 than in 2.  (Since the bromine atoms are pointing in opposite directions in 2, there is no steric strain in 2 due to interaction between the bromine atoms.)

see also torsional strain, angle strain, steric hindrance