To use the bibliography file you'll need to initialize and update the submodules associated with this repository. You may be able to do this nicely using a GUI. At the command line you'd do the following:
git submodule init
git submodule update
Some files are in Rmarkdown (.Rmd) or markdown (.md) formats. You can edit them using any plain text editor. For example, RStudio has a built in text editor. [more text editor suggestions]
If you want to turn Rmarkdown files into pdf files, you will probably want to install latex (for example, for Mac try https://tug.org/mactex/ on Windows http://miktex.org/ or https://www.tug.org/texlive/windows.html, for Linux https://www.tug.org/texlive/ ).