What does it mean in the 14th Amendment when it says that the public debt "shall not be questioned"?

What does it mean to question the debt and who, if anyone, is suggesting that we do so? This seems to be odd wording. It is not a matter of whether or not there is a debt. Everyone acknowledges that. It is a matter of whether the debt limit should be increased.

This is the related part of the 14th Amendment:
The validity of the public debt of the United States, authorized by law, including debts incurred for payment of pensions and bounties for services in suppressing insurrection or rebellion, shall not be questioned. But neither the United States nor any state shall assume or pay any debt or obligation incurred in aid of insurrection or rebellion against the United States, or any claim for the loss or emancipation of any slave; but all such debts, obligations and claims shall be held illegal and void.