A scientific theory a model that is able to make predictions about phenomena and also give an explanation as to why this phenomena occurs. Theories are thus backed up by evidence and they may not be, by definition, proven. They may, however be disproved at which time a new theory (either a new theory or an adjusted one of the previous model) will need to be put forward to explain observed phenomena.
Theories rank above laws in the scientific community which tends to lead to confusion amongst laymen whose definition of a theory and law is different to a scientific theory or law. Hence the non-sensical statement 'XXXXX is a just a theory'.