There are two different PINS entered in different places. Which one are you asking about?
The 6 digit IP PIN is entered under Federal / Other tax situations / Identity Protection PIN. The IRS issues an IP PIN to confirmed victims of identity theft.
The 5 digit e-filing PIN is required to electronically file your tax return. You choose a 5 digit e-filing PIN the first time you e-file, and use it every year thereafter.
If you don't remember it, you can use your AGI (adjusted gross income) from last year's tax return.