Questions about python algorithms.

https://www.hackerrank.com/challenges/python-mutations/problem

This is the question.

The answer is

def mutate_string(string, position, character):
    return string[:position] + character + string[position + 1:]

This is.

I don't understand why there is a position in the string [position + 1:] here. If I don't put this in, it's printed out like abackbracadabra, but if I put in position, why can't I print out the braca? Is that right to slice from the part where the position is located at 0 and then 1:?

return character.join([string[:position], string[position + 1:]])

And another answer is that in the case of something.join(a, b), there is an interpretation that something corresponds to something between a and b, but did I read it correctly?