The Ansible project uses GitHub (https://github.com) to manage code repositories, issues, and other aspects of the project. The Ansible organization (https://github.com/ansible) is where the code repositories can be found. The main repository is the ansible repository (https://github.com/ansible/ansible), where the core Ansible code, the modules, and the documentation can be found. This is the repository that should be cloned in order to develop a contribution.
The Ansible project uses a development branch named devel instead of the traditional name of master. Most contributions target the devel branch, or a stable release branch.