Git is a distributed version control system for managing software packages. Version Control is the 'practice of tracking and managing changes to software code'. When a software package is updated, one can create a new version which differentiates from older versions. One can also have different 'branches' of the versions representing different modifications or specializations.
The Thermodynamics system is stored on GitHub which is a general cloud repository storing software packages. This is where the latest version of the system can be found. Every time a change to the package is made, the changes are uploaded to the repository.
There are several ways to install Git on your system. Here are several tutorials, choose the one which is more understandable to you: