Basis would be an amount for which you did not receive a tax deduction when you contributed it. For federal purposes, contributions to a plan like a 401k were deductible, meaning they reduced the income on which you paid tax. For PA, though, that income used for contributions was taxable, so you had a basis in PA for those contributions. Hopefully, your 1099-R shows employee contributions. If not, you would have to get the information from the plan fiduciary.
Here's an article to read about it.
https://revenue-pa.custhelp.com/app/answers/detail/a_id/1470/related/1/session/L2F2LzEvdGltZS8xNTIxM...
