An artist's statement is a succinct explanation of your creative and professional goals as an artist; it should also provide the reader with a brief insight into what informs and inspires the art present in the rest of the portfolio. Generally an artist's statement runs from 1-5 pages, and many consider brevity to be an asset.
Artists' statements are most commonly used in fine art portfolios, particularly image-based portfolios within the fine art and photography fields, where work is not necessarily being created with a client in mind. Fine artists' statements tend to focus on how the artist's philosophies inform or reflect their work, as well as motivations for creating such art. Many artists include a statement for each series of work they produce, in order to help their viewer understand their specific choice of subject matter and medium.
In a portfolio, the artist's statement works largely to illustrate the larger goals of the artist. This allows a buyer and/or potential employer to see the potential directions the artist may take as they progress creatively.
"A Guide to Bio Writing for Performance Artists" by Jamie O'Reilly