Brand Ambassadors is are usually celebrities or public figure who are hired by companies to promote their brands or businesses.
The term is also loosely used for non-celebrity figures who are hired by agencies or companies to promote a brand by wearing their T-shirt or by using and displaying their products. Ideally, a good brand ambassador provides high visibility for the products and enhances sales.
Before choosing any brand ambassador, one must weigh the strengths and the characteristics of the brand ambassador. The personality representing the brand must have similar characteristics to the brand.
You can refer to a more detailed article here to understand about brand ambassadors and how to select them.
Great examples of brand ambassadors are Tiger Woods and Michael Jordan for Nike, Lionel Messi for Addidas, and closer to India, Juhi Chawla for Kurkure.