Hi Georges,
The first column is the gross salary that you receive from your employer. The second column is the amount that your employer pays to the tax office with each pay check they give you. These details appear in the payment summary (and payslip) that you receive from your employer. The idea is to be able to pay the right amount of tax at the end of each tax year.
So in the sample spreadsheet, Employer #1 paid you 100k in gross salary and 20k is tax withheld (If you work as a contractor, it is possible to have more than one employer for a give tax year). You might have 2 IPs generating a rental income of 12k and 15k. You don't have PAYG on rental properties so leave the second column blank for both properties.