[Tutor] Meaning of -1 in Python

[Ben Ganzfried]

I'm currently working through the Google Python tutorial exercises and
had questions about the following function:

def not_bad(s):
  # +++your code here+++
  # LAB(begin solution)
  n = s.find('not')
  b = s.find('bad')
  if n != -1 and b != -1 and b > n:
    s = s[:n] + 'good' + s[b+3:]
  return s

It's clear that n!=-1 and b!=-1 means something like : "if in the
string 's' we find the word "not" and in string 's' we find the word
"bad."

I'm wondering the following:
On a deeper computational level, what is going on here?  What exactly
does Python take -1 to mean?  Is it saying that since the string 's'
is indexed starting from 0 to len(s), and since -1 is not part of
that, that therefore something having the value of -1 will never be in
the string?  If so, then how exactly does using negative numbers to
count a string work?  I have read something about this earlier...

Also, does the part: b>n mean, in this case: "bad comes after not in
the string 's'"?

Thank you very much.

Sincerely,

Ben