The DCO is a representation by someone stating they have the right to contribute the code they have proposed for acceptance into a project: https://developercertificate.org
That representation is important for legal purposes and was the community-developed outcome after a $1 billion lawsuit by SCO against IBM. The representation is designed to prevent issues but also keep the burden on contributors low. It has proven very adaptable to other projects, is built into git itself (and now also GitHub), and is in use by thousands of projects to avoid more burdensome requirements to contribute (such as a CLA).
The DCO requires the use of a real name that can be used to identify someone in case there is an issue about a contribution they made.
A real name does not require a legal name, nor a birth name, nor any name that appears on an official ID (e.g. a passport). Your real name is the name you convey to people in the community for them to use to identify you as you. The key concern is that your identification is sufficient enough to contact you if an issue were to arise in the future about your contribution.
Your real name should not be an anonymous id or false name that misrepresents who you are.