I'm showing a taxable distributions of $556.00 on my "your HSA summary" in Turbotax. How is that amount derived?

Did you enter information from Form 1099-SA for a distribution?

On the HSA Summary:

Taxable distributions -The amount that you took out of the HSA but not for qualified medical expenses. This amount is added to your income on line 21 (Other Income) on Schedule 1 (Form 1040).

As Annette noted above, this happens when you enter a 1099-SA but do not indicate that the entire amount of the distribution was for qualified medical expenses.