This tax form is essential for expats arriving in The Netherlands.
Did you arrive during the course of a year, e.g. 2018; you need to fill in this form and send it to the Belastingdienst.
The M form is your first introduction to the tax system in The Netherlands.
If you arrive during 2018 you will need to fill this in for the tax year 2018. You have 5 years to complete. After requesting the form at the tax authorities and the Belastingdienst approve your input, you will fill in the next tax year, e.g. 2019, in a digital manner.
If you need tax deductions in the future, e.g. you want to buy a house, you have to start your input a.s.a.p.