It is asking for any carry-over contributions you might have. If last year you made a contribution over the limit and the credit was limited, the excess contributions would have been carried over.
On that TurboTax screen where it's calculating your State tax deductions based on your 529 plan contributions, there are three columns (something to this effect): Beneficiary Name, Contribution Amount, Unused Amount. The Contribution Amount you should be able to see online with your 529 provider website or in the statements you receive by email/mail. That's the total amount you contributed to each beneficiary's account last year. I live in Ohio, and use their 529 Plan. Their plan limit is $4,000 for tax deduction purposes (you can put more in, but only $4,000 is tax deductable). So for example, if you contributed $5,000 to Child A, you put that amount in the Contribution Amount column on the TurboTax screen, and the $1,000 in the Unused Amount field (because $5,000-$4,000=$1,000). Since there is no in-line screen help for 529 Plans in TurboTax, I had to use the Chat agent for help and read thru the Ohio 529 Plan help screens to figure this out.