The workflow says that the master branch of a project is the golden branch from which all development is based off of.
In order to keep this easy to navigate, it is asked that you squash your commits down to a few, or one, discreet changesets before submitting a pull request. Fixing a bug will usually only need one commit, while a larger feature might contain a couple of separate improvements that is easier to track through different commits.
Once you have rebased your work on top of the latest state of the upstream master, you may have several commits related to the issue you were working on. Once everything is done, squash them into a single commit with a descriptive message, like "Issue #100: Retweet bugfix."