Your signature ID (also called a self-select PIN) is a random 5-digit number you entered last year, right before you transmitted your return. You can use it to verify your identity on this year's return.
It's not the 5-digit e-filing PIN you may have gotten from the IRS last year, unless you chose to use the same 5 digits as your signature ID (which is perfectly fine).
We don't store your signature ID in your account and it's not on your printouts. Hopefully, you memorized it, wrote it down, or took a screenshot of it. If you forgot last year's signature ID, you'll need to provide your 2018 AGI for verification purposes.
You'll be able to use this year's signature ID to verify next year's return, so make sure you record it.