For Loops & Variables

[aditvaddi]

for i in range(3):
    for j in range(2):
        print(j)

I don't understand how the output of this is:
0
1
0
1
0
1

-why do those variables contain these integer values already?
Thats the only part i really dont get. I understand that for every loop in i, 2 loops of j play out, that why for each loop, the value generated is first 0 then 1 but where did these values come from?