Brand guidelines are a clearly defined set of rules and standards that help to communicate how your brand should be represented. By having brand guidelines, businesses are able to ensure consistent messaging in everything that they do, by demonstrating what your company is, what you do, and what you stand for.
A brand guide is often referred to as a brand bible, and covers everything from how and where to use your logo (or logos), which fonts and typefaces to use and when, along with a clearly defined colour palette.