A brand mantra is a short, three to five-word phrase that captures the irrefutable essence or spirit of the brand positioning. It’s similar to “brand essence” or “core brand promise,” and its purpose is to ensure that all employees and external marketing partners understand what the brand most fundamentally is to represent to consumers so they can adjust their actions accordingly.
Brand mantra consists of three parts, brand function, descriptive modifier, and emotional modifier. The brand functions term describes the nature of the product or service or the type of experiences or benefits the brand provides. The descriptive modifier further clarifies its nature. The emotional modifier provides another qualifier "how exactly does the brand provide benefits and in what ways?"
Source: Strategic Brand Management 4th Edition - Kevin Lane Keller