How much should be explained in stories?

When you write a story, how much details should be explained? I mean, like how technology works, details abouit planets/species and so on. Because for me, sometimes I feel like I am not explaining enought, but then I am starting to explain everything and it disrupts the flow of the story. So, how much do you thinmk should be explained in story? And I am specifically asking for the narration of the story, not appendixes/supplementary material.