The answer is partly in the name of the theory. The Endosymbiotic Theory, endo meaning "to absorb" or "to contain" and symbiotic meaning "a mutually relationship between two bodies in which both mutully benefit from" or "close physical interaction between two organisms".
The theory suggests that at some point in time, a prokaryotic cell was consumed by another cell which eventually led to the development of organelles such as mitochondria and chloroplast. The strongest evidence for this statement is the fact that these organelles have their own DNA, which can be an indication to the fact that once they were seperate organisms.
