Three characteristics of a good scientific theory: testable, repeatable and stable.
Testable
Must make predictions; must be falsifiable.
Testability, even more than falsifiability, is probably the most fundamental aspect of science, separating it from theology, maths and philosophy.
To be truly testable, a hypothesis should be falsifiable, with counter-testing and proof of the null hypothesis possible. - Explorable.com
Repeatable
Others get similar results.
Data are reliable, not faked
Stable
Theory unchanged after repeated testing
Scientific observations are the fuel which power scientific discoveries and scientific theories are the engine. Theories allow scientists to organize and understand earlier observations, then predict and create future observations. Scientific theories all have common characteristics which differentiate them from unscientific ideas like faith and pseudoscience.
Scientists don't use the term "theory" in the same way that it's used in the vernacular. In most contexts, a theory is a vague and fuzzy idea about how things work — one with a low probability of being true. This is the origin of complaints that something in science is "only a theory" and thus isn't credible.
Learn more about the criteria for science and scientific theories at ThoughtCo.com.