The confusion over the meaning of the word "theory" is frankly one of the worst things to happen to science. The colloquial use of the word to mean "I have an unsubstantiated feeling about something" has lead to a unified cry of "it's only a theory!" from everyone from climate change deniers to creationists as a way of casting doubt. "Theory" in a scientific context is basically a way of describing observable, demonstrable facts. To get a theory, you begin with a hypothesis (which is probably what most people mean when they say "theory") and you test it over and over again until you have enough evidence to support your hypothesis, at which point it becomes a theory. Theories are not the same as laws, but they are used to describe laws. A law is something that you can observe, for example, that when you let go of an object on Earth, it will fall to the ground; the Theory of Gravity explains why that is so. To understand why something like evolution is not "just a theory", you have to give it its full title: The Theory of Evolution by Natural Selection. The "natural selection" bit is actually the theory part. We know that life on Earth evolves, that is an observable fact. The theory part of it is that it does it by natural selection, which has been tested and found to pretty accurately describe the patterns that we can observe in evolution.
