Before patches can be accepted, contributors must sign the Google Individual Contributor License Agreement (CLA), which can be done online. The CLA is necessary since contributors own the copyright to their code, even after it becomes part of the codebase, so permission is required to use and distribute that code. Contributors don't have to sign the CLA until after a patch has been submitted for review and a member has approved it, but the CLA must be signed before the patch is committed into the codebase.
Contributions made by corporations are covered by a different agreement than the one above, the Software Grant and Corporate Contributor License Agreement.
See also: https://cla.developers.google.com/