Popular within many open source projects but not only.
Master branch of a specific location (Github, Gitlab, Bitbucket, local server) contains the latest shippable version. For each new feature/bug fix/architectural change each developer creates a branch.
Changes happen on that branch and can be discussed in a pull request, code review, etc. Once accepted they get merged to the master branch.
Full flow by Scott Chacon:
Originally presented on Scott Chacon's personal web site.
Image courtesy of the GitHub Flow reference