You are given a string that you have to parse into tokens, which you can see in the "Token Grammar". For example:
str = 'return "String" + "number"'
From the table you must map each part of the string to a token from the table
result_tokens = 'return' - keyword; ' ' - whitespace; 'String' - string; ' - whitespace; 'number' - string
In addition, you need to implement an iteration protocol.
I hope this makes more sense to you.