A Product Brand is used to add extra information to your Products. Once a Product Brand is created, it will then appear in the 'Brand' field when creating or editing a Product (How do I add a new Product?)
To create a Product Brand, choose Manage > Products > Brands
To add a new Brand, click +Add Brand
Enter the name of the Brand, and click Save
For information of how to add a Brand to a Product, see: How do I add a new Product?