What does it mean to be 'On branch master'?

Being "on branch master" means that your current working directory in a Git repository is set to the master branch. This indicates that any changes you make, commits you create, or files you modify will be applied to the master branch. The master branch is typically the default branch in a Git repository, representing the main line of development.

You can check which branch you are currently on by running the command:

git branch

The output will show a list of branches, with an asterisk (*) next to the branch you are currently on, like this:

* master
  feature-dimension