From the IRS:
An identity protection PIN (IP PIN) is a six-digit number that prevents someone else from filing a tax return using your Social Security number (SSN) or individual taxpayer identification number (ITIN). The IP PIN is known only to you and the IRS. It helps verify your identity when you file your electronic or paper tax return. Even though you may not have a filing requirement, an IP PIN still protects your account.
If you don't already have an IP PIN, you may get an IP PIN as a proactive step to protect yourself from tax-related identity theft. Anyone with an SSN or an ITIN can get an IP PIN including individuals living abroad.
For more information click on the following IRS link: https://www.irs.gov/identity-theft-fraud-scams/get-an-identity-protection-pin
For steps on how to get an IP PIN watch the following YouTube video.