A Letter of Intent is a letter from an Operator indicating their intention to work with a vendor in a given jurisdiction. This is basically a precursor to a contract. States require a Letter of Intent because regulators can’t spend time on a company's license application if that company does not have a realistic chance of working with an Operator in that state.
Currently the Michigan - Vendor Registration Application and West Virginia - Sports Wagering Interim License Application require a Letter of Intent. Typically, the Operator will have a form they will send over to you.