Git and Github are designed for versioning code, using it as a versioning platform for hardware files is a bit out of the scope this tools are made for.
Therefore, we use quite advanced git features such as Submodules and Git LFS.
To clone this repository, you need both Git and Git LFS.
Source Tree is a software providing a nice and easy to use interface for Git. If you are not familiar with Git, it helps a lot understanding what the hell happens.
You can download it here
Once you have installed every soft needed, you can "clone" the repository.
Configure your app, then click on the new repository button and specify the url of this repository https://github.com/poppy-project/poppy-humanoid.git
If you have forked this project, replace the url by your https://github.com/YOUR-USERNAME/poppy-humanoid.git
Open your terminal (or Git Bash on Windows) and tape git clone --recursive git://github.com/poppy-project/poppy-humanoid.git
This will clone the entire repository including all submodules and download all files.
Again, if you have forked the repository, change the url accordingly.