Intersex is a term referring to people whose sexual characteristics don't fit within the societal norm of male and female. These characteristics might be visible at birth, or may only develop during puberty.
The intersex flag was designed in 2013 by Morgan Carpenter, Executive Director of Intersex Human Rights Australia. Morgan sought to create a flag that wasn't derivative of existing pride flags, but to instead create something new but still grounded in meaning. The colors yellow and purple are often used in other pride flags to represent those outside of gender norms, such as in the Non-binary flag. The circle in the centre is unbroken and unornamented, complete as it is. It's a statement of who intersex people are, and who they intend to be.
Sources and acknowledgements
The Intersex flag design is in the public domain under a CC0 1.0 Universal (CC0 1.0) declaration.