Can you explain the meaning of "theory" in science? Is it accurate to say that a theory is simply an assumption and cannot be proven to be completely correct?

This is, actually, a most interesting question, and has implications for all of science, currently, and historically. It is profoundly ironic that two of the most noted “theorists” in the history of physics — Galileo and Newton — expressed hostility to the very concept of theory, and did not consider themselves to be “theorists”, at all! Both Galileo and Newton profoundly disliked the ancient, clockwork, theoretical conceptions of the universe in which everything was completely understood, and had its own proper and inevitable place. They saw the universe as infinitely mysterious, and saw their work as simply adding certain observations of interest to it, not as providing comprehensive theoretical models, at all. Nevertheless, the field of physics did indeed treat their work as being a sacrosanct, all inclusive theoretical conception. All we can have in science is data from observations, nothing else. However, the powers that be rather like simple, all inclusive explanations of things. So, there is a tendency for honest, brilliant scientists like Galileo and Newton to have their observations and work twisted to meet the needs of the powerful, for simple explanations of everything, as a control strategy, to accomplish their political agendas. And, there is a tendency for “politicians”, like Albert Einstein and Charles Darwin, to have even their most ridiculous conceptions, like the theory of relativity and the theory of evolution, to be adopted as Gospel, simply because they appear to provide simple explanations for everything, and for “evidence” to be manufactured supporting them!