This is the part of the plot that sets up the story. You, as the author, can tell anything to your audience that you wish them to know before the story gets too far underway. Common things are to introduce characters, state the setting, give conflict details, and to give backstory about prior events. There are two principal ways to achieve exposition, information dumps and incluing.
An information dump is the messy, straight to the point expository style that does what it says and drops many things on the reader at once. This type of exposition can work well under certain conditions, but avoid its use when you can. If you intend this type of exposition, set it up to come across as a story within the story… Like a soliloquy in a Greek tragedy.
Open with a monologue by one of your characters and have them tell what you need, don’t just shove a sandwich at your reader. Describe the taste of the sour pickles, the tanginess of mayo when combined with the sweetness of tomato, the sinewy texture of the roast beef in contrast to the softness of the fresh bread. Add the salt and pepper to spice it up, use sharp cheddar instead of processed cheese slices.
The other form of exposition is incluing, which slowly brings the reader onboard with necessary information. We attribute the word to author Jo Walton, who defines it as “the process of scattering information seamlessly through the text, as opposed to stopping the story to impart the information.” We can achieve this through dialogue, flashbacks, in-universe media, stream of consciousness, narrated backstory, or by giving background details.
The entire purpose of incluing is to clue the reader in without telling them they are learning about your universe. It makes for a more engrossing read that will GRADUALLY drag the reader in without boring them. Through several chapters, you can tell the entire backstory instead of throwing it all in the first few paragraphs of your opening.