Just because you know something doesn’t mean your reader knows it, too. This is often an issue if your story begins with dialogue. Suppose you open your story with the following exchange:
“What the hell do you think you’re doing?”
“What do you think I’m doing? It’s not exactly a state secret.”
“Well let me tell you what you’re not going to do. You’re not going to lie to me any more.”
You may want to draw the reader in by starting with an intense argument, but the reader is completely in the dark. Who is talking? What is their relationship? Are they face to face? On the phone? We don’t know whether there are two or even three speakers, whether they are men, women, or at least one of each. The exchange would be interpreted very differently if the speakers are husband and wife as opposed to mother and daughter as opposed to co-workers, etc. You get the idea.
Give the reader enough information up-front to set the scene, to allow the reader to put the exchange in contest. Otherwise, the reader is likely to give your story a pass.