I do not understand the "tax on excess accumulations in retirement account." Is this additional tax I owe? And if so, why?

The tax on excess accumulations in retirement accounts is an additional tax you would owe if you made an excess contribution in a prior year.  Because IRAs have tax advantages, the limits must be adhered to- for example, if I ignored the contributions limits and put $100,000 in a ROTH IRA, I would be able to have more tax-free income on the earnings when distributed.  It is 6% per year that the excess is kept in the account to encourage correction.  

It is be in addition to any tax you paid on your IRA distribution and/or tax for late payment of RMD.  

Please see Tax on Excess Contributions.

It also could be if Turbo Tax thinks you didn't take your full RMD amount.  Did you have more than 1 1099R?  Go back though the 1099R and followup questions.  It will ask how much is your RMD.  It usually should be the same amount as that 1099R box 1.  It asks for each 1099R separately so don't enter the total RMD for all.