Everything is already written (and better than we can do here) but for those who are too lazy to read in English, here is a very short summary.
An excel file, in Python, can be transformed into a workbook object, which contains the worksheets (like an excel file contains many sheets or sheets). Actually, even though it is not a very common name (you will probably be used to the word spreadsheet), Workbook is in fact a synonym for Spreadsheet. So, when you load a Workbook object, what you are doing is nothing more than transforming your excel file into a Python object (with all its properties).