Git is a command line application, meaning it's an application that we run in the terminal / command prompt.
The purpose of git is to keep track of our works. As a programmer, what we usually do at work is write code. With git, we can see what are the things that we added, edited, or removed from our project's source code.
By default, git doesn't just track everything we do to our source code. Therefore we need to tell git which changes we made to the source code that we want to keep track later, as we discuss later.