A brands message is a clear and concise message aimed at your target audience. It can be delivered in anything from a slogan or tagline to a mission statement or vision statement. What you say, has to be true for your brand and of interest to your audience.
The purpose of having a core message is to make sure that everyone understands the same thing about your business so that they can all work towards the same goals. This helps reduce confusion and ensures that everyone is moving in the same direction. This applies just as much for people within the business as to external partners such as customers or other stakeholders.
Having a clear, concise message helps create consistency within your brand, which is vital for creating an emotional connection with potential customers or collaborators.