Brand guidelines are a visual roadmap for how to use your brands visual elements. They are used by designers, marketers, illustrators, copywriters and anyone who needs to use your brand to create marketing materials.
Typically, these guidelines were bound into a book, but nowadays, you'll more likely to find them in a PDF format.
They'll show you how all your different logos, icons, typefaces, fonts, colour palettes, additional graphics work together.
But the most comprehensive ones also include your brands strategy. Including mission and vision statements, USP's, target market personas, brand story, personality and tone of voice.
In my opinion, a set of brand guidelines include your brand strategy and visual design elements – this way, when you want to introduce a new service or product, you can see how it fits alongside other parts of your business without having to run back to both the brand strategist and the graphic designer.