What's "so" mean and how's it used here?

Native speaker’s answer
Rebecca
Here, "so" is an adverb that refers back to what was previously said and has a similar meaning to "true." The phrase "Is that so?" is asking, "Is what you said true?" Ex: Is that so, Ryan? Did you really lose the keys? Ex: A: I don't like pineapple on pizza. B: Is that so? Then why did you order it?