A mission statement is a declaration of the purpose of a company, organization, or person; it's reason for existing; a written affirmation of an organization's core purpose and focus that generally remains unchanged over time. It identifies the goal of its operations, what kind of product or service it provides, it's primary customers or market, and it's geographical region of operation.
It may also include fundamental matters, such as the organization's values or philosophies. The business's main competitive advantages, or it's desired future state, which is often referred to as it's "vision." It's an expression made by its leaders.