A theory is a suggested explanation as to why something exists as it appears to exist, or behaves as it appears to behave. Good theories are, therefore, based on consideration of as much evidence as possible.
For example, there used to be a theory that the giraffe has a long neck because it, and its parents before it, stretched their necks to reach high leaves and this effort led to changes in the length of their necks, which could be passed on, to the next generation.
As further evidence has come to light, over time, this theory has now been replaced by the theory of evolution.
It is possible to see from this example, however, that all that we call "scientific knowledge" actually consists of theories, which are being continually refined by further research, but which remain theories, all the same!