I have seen this mentioned a few times and have no idea what it is. I get the impression that it is referring to the combination of the Cardano Foundation, IO Global and Emurgo.
These are synonyms. Stiftung is German for foundation. The Cardano Foundation is located in Zug, Switzerland - a German-speaking part of the country.
The twitter handle of the (now verified) Cardano Foundation is CardanoStiftung: https://twitter.com/CardanoStiftung.