Answer:
Many open source projects will only accept patches (code contributions or documentation contributions) from people who have submitted a legal document known as a contributor agreement. Contributor agreements are not open source licenses — rather, they are a way for the contributor to tell the project that it has the right to distribute the new contributions under the project's existing open source license. (Some contributor agreements also allow for the project to distribute the contributions under other open source licenses too, which enables projects to change their license in the future, and some agreements even allow the project to distribute the contributions under any license the project wants.)
There are two kinds of contributor agreements. In a Contributor License Agreement (CLA), the original contributor retains copyright ownership of their contributions, but grants the project a broad set of rights such that the project can incorporate and distribute the contributions as it needs to. In a Copyright Assignment Agreement (CAA), the contributor actually transfers copyright ownership of the contributions to the project, who can then license it however they want since they own it (but a CAA typically grants very broad non-exclusive rights back to the contributor so that they too can use, distribute, sublicense etc their contribution freely).
With both CLAs and CAAs, it is of course necessary that "the project" be some kind of legal entity able to enter into agreements. Sometimes the project is incorporated itself, usually as a non-profit entity; sometimes it is represented by an umbrella non-profit organization (such as the Apache Software Foundation or the Software Freedom Conservancy); sometimes a for-profit corporation considers itself the main sponsor of the project and requests contributor agreements in order to manage the development community and maintain a public distribution of the software in question.
For more about contributor agreements in general, and some examples, see civiccommons.org/Contributor_Agreements. See also the Project Harmony, "...a community-centered group focused on contributor agreements for free and open source software (FOSS)."