I don't understand this, what does this do?

Question

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

print(func(4))

Answer 1

Maybe python would be a good tag for the question.

Answer 2

func(x) gives the sum of numbers from 0 to x-1.

Answer 3

For future reference, please tag a relevant language to improve context clarity ... ☝