A standard operating procedure sets out who does what, in what order and to what standard, for a task that comes up repeatedly: onboarding a client, handling a returned item, closing the month. It captures knowledge that otherwise lives in people's heads.
Good SOPs are short, specific and kept current. They make training faster, reduce variation between people and sites, and form the basis for automation, because you cannot automate what you have not defined. They also make a business easier to understand for anyone outside it, including a prospective buyer.