PR is short for Pull Request. There's some good documentation about that on Github (better than what follows
), but to summarize:
* create your own Github account, if you don't have one yet;
* fork the Cesium repository from Github to your account;
* `git clone` your account's copy of the Cesium repository;
* modify whatever you want to modify;
* select files for addition to the commit through `git add` or one of the GUIs;
* create a commit through `git commit` or one of the GUIs;
* `git push` (from the CLI or through one of the GUIs, again) to your repository;
* click on the pull request creation button from the Github Web interface when browsing your account's copy of the Cesium repository where you have just pushed.
Of course, for trivial changes, the process may be a bit heavyweight.