Why does “cd -” behave as it does?

What does cd - exactly do ? ( the change directory command, plus a dash )

I noticed that if I run it in my /home/user folder repeatedly it outputs either /home/user or /home, this changes if I run it from a different folder.

Answer

cd - 

pop the last directory you were from the stack of directory. It’s like hitting “back” on the browser.

Exemple :

you are in /user/alex

you can test that with : pwd that give you /user/alex

then if you do

%cd project1/subfolder
%pwd 
/user/alex/project1/subfolder
%cd subsubfolder
%pwd
/user/alex/project1/subfolder/subsubfolder
%cd - 
pwd 
/user/alex/project1/subfolder
cd - 
pwd 
/user/alex

NB : it’s not going back a level upper in the folder hierarchy. it’s going to the previous current folder. ( a level upper is cd .. ).