A pull request is a request to add or change a line of code in a shared repository. If you are making a change into your own repository, you would make a commit. The difference between a pull request and a commit, is the approval process. A pull request requires approval from the project owner and the maintainers of the project. A commit requires approval from the project owner, which would be you for a project you created or forked.
A repository is a collection of files and code that can be shared and modified, if the license permits, for your own needs. Repositories can be hosted on GitHub, such as this one is, or other hosting services. They include the history of changes over time, so you can go back to a previous version of a file if you need to.
All ideas for what to do 100 of, are licensed under CC-BY-SA-4.0. This means that you can use it for any purpose, and modify it, but you must give credit to the original author.
All linked works remain under the original license of the work.