"2b has checked 'Taxable amount not determined'. What does box 2b really mean?"
This means that the IRA custodian doesn't have all the information necessary to know how much of this distribution is taxable. For example, there may have been a possibility that you contributed after-tax dollars (non-deductible) to the IRA in previous years which would have not been taxable when returned to you in the distribution, but the IRA custodian's records are not complete enough to determine this.
As the IRS instructions for form 1099-R say,
"[the IRS custodian] are not required to compute the taxable amount of a traditional, SEP, or SIMPLE IRA nor designate whether any part of a distribution is a return of basis attributable to nondeductible contributions."
For IRAs, the IRS's instructions say to put the same amount in box 2a as was in box 1, and then to check the 2b box 'Taxable amount not determined'. I believe that this is a reminder to you to check the basis (amount of non-deductible contributions in the IRA) for yourself.
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