Quick Definition: A testable theory can potentially be disproven through observation or experiment to be considered scientific
From Rational Wiki:
Falsifiability is the ability of a theory—a working framework for explaining and predicting natural phenomena—to be disproved by an experiment or observation. The ability to evaluate theories against observations is essential to the scientific method, and as such, the falsifiability of theories is key to this and is the prime test for whether a proposition or theory can be described as scientific. Put simply, if a theory cannot be falsified, there is no point in even examining the evidence.