Your brand message is the one thing that ties together every single piece of content that you create. It is the underlying value proposition, and language, that you communicate in your content. For Apple it is "think different", for Nike "just do it"...
It is your slogan - but it is also a lot more than that.
Your brand message should include everything you know about your audience, your competitors, your target audience, your mission and your tone of voice. It should be a positioning statement - one that defines where you fit in the marketplace.
Your message should be simple, but the work and thinking behind it should be complex.
Together, these elements combine to create a full messaging framework that can guide your marketing across every piece of content and every channel that you want it to be on.
Everything you create, both internally and externally, should be able to map back to this messaging.