"this is a matter of freedom, not price, so think of 'free speech,' not 'free beer'"
Definitions via https://www.gnu.org/philosophy/free-sw.en.html
The freedom to run the program as you wish, for any purpose (freedom 0).
The freedom to study how the program works, and change it so it does your computing as you wish (freedom 1). Access to the source code is a precondition for this.
The freedom to redistribute copies so you can help others (freedom 2).
The freedom to distribute copies of your modified versions to others (freedom 3). By doing this you can give the whole community a chance to benefit from your changes. Access to the source code is a precondition for this.
I Am Not A Lawyer
Copyright: Only the author/creator has rights to copy, distribute, adapt the work
Copyleft: the public retains freedom to copy, distribute, adapt the work, provided it is under the same copyleft terms
Not technically a license!
Creative Commons comes in here
Not necessarily Git-based