On a pay stub, what is the difference between net pay day and ytd net pay

The difference is: Net Pay is how much you've made during a pay period, YTD Net Pay is how you've made this year.

on a pay stub, YTD stands for year-to-date. 

In some cases, both of these may not be equal because not all organizations have the same financial period (most of them choose one year as the financial period, but plenty of them choose less, like 4-month or 6-month per one financial period)