Recursion on string: What does the line 'return s[0] == s[-1] and isPal(s[1:-1])' do?

Well you have to think through this step by step:

1. You pass abcba to isPalindrome
2. isPalindrome calls isPal(toChars(s))
3. toChars(s) returns "abcba" so this is passed to isPal(..)
4. isPal is called with the argument "abcba".
5. Check: is len(s)<=1? No, len(s) is 5.
6. So to else: is s[0] == s[-1]? Yes. If it wasn't, this function would stop right here and return False. But now to the next step.
7. Since s[0] == s[-1] is True we need to evaluate isPal(s[1:-1]). Keep in mind, that s[1:-1] is now "bcb". So run isPal("bcb").
   1. len("bcb") is 3 so go to else.
   2. s[0] == s[-1] is True. Evaluate isPal(s[1:-1]) where s[1:-1] now is "c".
      1. len(s) is 1, therefore: return True
   3. isPal(s[1:-1]) returned True so s[0] == s[-1] and isPal(s[1:-1]) is True. Return True.
8. isPal(s[1:-1]) returned True so s[0] == s[-1] and isPal(s[1:-1]) is True.
9. isPal(toChars(s)) returns True: You have a palindrome!

Hope this makes it clearer for you.

EDIT Step 6 does always come before Step 7 because Python explicitly goes from left to right in logical expressions, see here. If this does not happen, your interpreter is broken.