If you have a withdrawal from a Roth IRA, it can be taxable in certain situations. For example,
1. You are withdrawing earnings, and you are either under age 59-1/2, or it is less than 5 years since you opened your first IRA.
2. You are withdrawing a conversion that happened less than 5 years ago, and you are under age 59-1/2.
(Remember that Roth withdrawals are always contributions first, conversions second, and earnings last.)
If you are in one of those situations, you need to certify to the IRS (by signing your return) that you are only withdrawing your original contribution basis. If you are over age 59-1/2 and the account has been opened more than 5 years, Turbotax shouldn't even ask the question and you can make a reasonable guess if you are forced to enter an answer. If you are in one of those situations, you should provide as accurate information as possible in case of audit.