Social canines typically display this behaviour when interacting with another they know well. The more self-confident dog will muzzle grab the insecure dog as a way of asserting a social position.
Mothers use muzzle grabbing as a way to deter puppies from suckling during weaning and in turn the puppies learn through play that it is a safe way of stopping a littermate from doing something, learning bite inhibition in the process.
This behaviour does not involve biting, simply grabbing. It is a behaviour which helps to develop trust between both parties as they communicate self-control and a non-combative intent.