Found in the book "Mortgage Free!" by Rob Roy, this is a visualization of how many days of each year you are working for a expense(such as taxes or housing).
in other words, imagine all of your earned income went towards a yearly expense at the beginning of the year. This helps you visualize the percentage of your total income you are paying.
Reccomended inputs are taxes and housing costs, but other costs like utilities or anything can be used as long as you know the yearly cost and your income.