I made this for beginners in coding and computers.

Proprietary Software
You are NOT allowed to access the code (At least not without breaching copyright law)
Examples most software you have to pay for.

Source-Available Software
The copyright holder lets people edit CERTAIN parts of the code.
An example includes Epic Game's Unreal Engine.

Open-Source Software
You are legally allowed to access the code of Open-Source Software.
A great example is all the projects on this website! (That is one of the reasons why people can remix your projects)

Intresting.