For example, if a British company earns income in Germany, its pays German taxes on its German income. But if a US company earns income in Germany it pays both German taxes and US taxes.
What is little known about this tax bill is that it normalizes our tax system with the rest of the world by moving to a territorial tax system. This is not about Apple avoiding tax on profit earned in the US, they will continue to pay US taxes, they won't pay taxes on income earned outside of the US going forward. [0] That means that there won't be any more hordes of overseas profits. And means the end of those tax inversions or corporate inversions you have been hearing about. [1]
So now that foreign profits aren't going to be taxed, something needed to be done with all the profits generated under the old system. The 23% repatriation tax is a compromise between the new rate of effectively 0% and the old rate of 39%. This BTW happens to be very close to the OECD, the developed world's, average tax rate of 24%.
Edit: As was astutely pointed out below the US only pays additional taxes to the US if their US tax bill was higher than their German one. And they paid the difference between to two to the US.