A style guide is a document that includes information about different graphic design elements for your business. It could start simply with the electronic version of your business logo. Additional things to include are the color values used in your logo and the font. Then, it could include all color and font values used in your marketing and operation materials. Finally, it could include any graphic elements that you use in your business.
The benefit of a style guide is that it forces you to organize all the graphic elements for your business. Then it can be shared with anyone in your business who is producing graphics so things are uniform.
