One thing it can be tempting to do is to try and explain everything, to make sure everything is clear. This can be bad.
For example, if I used the phrase, “Don’t be a grammar Nazi,” in a situation where a reader might not be familiar with it I might give a brief explanation, but I could hardly explain what a Nazi was quickly. You could write libraries about what Nazi’s were, what they did and what was done to stop them.
You have to know when the stop, some things you have to leave hanging because they simply are too complicated and too lengthy to go into.
Other things you leave hanging because it helps with worldbuilding. For example in Star Trek it was common for one of the main characters to say “It’s like A, B, or X.” Where A and B were known to the viewers, and X was alien.
This helps make the world feel real, it also can leave you places to build in the future. If you say there was a war between two planets you don’t have to say why it was fought, you can define that later.