index is the actual position of the character in the sequence, generally index in programming language is start with the 0.
here number '5' is at index '0'
and number '8' is at index '3'
positive index is valid in all programming languages but negative index is also supported by the python programming language.
Negative index -
so here number '5' is at index '-1'
and number '3' is at index '-3'