You're probably going to have to read Karl Popper's paper on Falsification theory. The answer that LostPraxis gives explains it really well but I figured you might like some examples taken from Karl Popper's actual works.
During this time ideas like Communism and Psychoanalysis were all the rage in the world of economics and psychology respectively. Popper made the criticism though that these theories had no way of being disproven and therefore should not be accepted as theories at all.
No matter what you do, you could always look at some injustice in the world and contribute it to class inequality. It's a broad idea. Yet, when Karl Marx was first proposing Communism he had very specific observations that he was going to use to show that Communism was correct. For example, he thought that Communism was going to first happen in very first class economies like England. This didn't happen, Communism first hit Russia a place not too far away from the Dark Ages. Yet when this idea turned out to be not true people revised the theory of Communism to just mean, "Class inequality causes problems." This is too vague to be considered a scientific theory and therefore should be disregarded.
Compare this to Einstein's theory of relativity. Almost all parts of this theory are ultra-specific and have very visible grounds for being proven or disproven. For example, light is a particle and gravity has an effect on light.
To prove this they took pictures of the stars in a certain segment of sky during an eclipse where the sun's gravity should pull the light from the stars closer to the Sun. Then they took a picture of the stars in that segment of the sky when the Sun should have no affect on the starlight. Well, the gravity did affect the light and this helped prove Einstein's theory of relativity. Popper helped with this experiment oddly enough, he even mentions it.
So what Popper is saying is that if you have a theory that can be falsified with experiments then you have an acceptable scientific theory. If you have a theory that cannot be disproven no matter what kind of experiments you do to it, that is not a scientific theory. Like comparing Communism to Quantum Physics.