You can pick up a marketing textbook or a scholarly whitepaper on the topic of marketing and get a much more complicated definition, but this one from Hinge Marketing just nails it for me:
To be meaningful, a brand must be “for someone.” No brand can try to be everything for everyone and hope to be successful.
It's not a complete definition, that's true. But it's the key to relevance and success.
I am not a surfer. Surf clothes and shoes with ultra-cool brands are just not meaningful for me. But that's ok because they are not for me. They are for surfers and surfer wannabes.
Surf brands exclude me on purpose, and that's a good thing.
Some surf brands, Ocean Pacific for example, have escaped the confines of the surfer market because they are sold in Walmarts everywhere. But I am not talking about Ocean Pacific here.
Knowing who your brand is for, knowing whom you serve by being who you are and doing what you do is the heart core of successful branding.