why is this code's output is 6

Question

def func(x):
res = 0
for i in range(x):
res += i
return res

print(func(4))

Answer 1

So res = 0
x = 4
range(x) = range(4) which is 0, 1, 2, 3
0+0=0, 0+1=1, 1+2=3, 3+3=6
I hope you understand now

Answer 2

The function sums all the numbers between 0 and x-1.

Answer 3

If you dont understand functions:
func(4) calls func(x) and sets x to 4.