In chemistry, a molecule experiences strain when its chemical structure undergoes some stress which raises its internal energy in comparison to a strain-free reference compound.  The internal energy of a molecule consists of all the energy stored within it.  A strained molecule has an additional amount of internal energy which an unstrained molecule does not.  This extra internal energy, or strain energy, can be likened to a compressed spring.  Much like a compressed spring must be held in place to prevent release of its potential energy, a molecule can be held in an energetically unfavorable conformation by the bonds within that molecule.  Without the bonds holding the conformation in place, the strain energy would be released.