If your friend buys her jeans based on the brand, it means that the name on the label of her pants is very important to her. A brand is a product name.
The cereal aisle at the grocery store is a good place to see how many different brands there are of the same product. A company name, or the name it invents for a particular item or service, is a brand. Another kind of brand is the verb that means to mark or name something in a negative way. If you brand your classmate as a bully, you accuse him of bullying and expose his behavior to other people.