There is a sign in Tempelhof airport in Berlin. What does the abbreviation RTF mean? I tried to google but found nothing.
"RTF Instructions" probably means "Radiotelephone Instructions"
As suggested here, it should not be confused (as I initially did) with the "RTF" of internet fame (from RTFM, for those that are still wondering after reading the translation below), meaning something completely different:
Ophelia und ich, die beide mehr Ahnung vom Internet als von der Luftfahrt haben, konnte hier drüber lachen: Hold for Follow Me or read the f[-]ing instructions. Tatsächlich wird es wohl bedeuten, dass der Pilot entweder das Follow Me-Fahrzeug abwarten soll oder den Anweisungen des Towers folgen, die über Sprechfunk (Radiotelephone = RTF) vermittelt werden.
Ophelia and I, both of whom are more familiar with the Internet than aviation, could laugh over it: Hold for Follow Me or read the f[-]ing instructions. In fact, it will probably mean that the pilot should either wait for the Follow Me vehicle or follow the instructions of the tower, which are transmitted via radio (Radiotelephone = RTF).
This is also confirmed by this source where "RadioTelephone (RTF)" is listed in several occasions.
Context in terms of location would be helpful, but it could mean "Return To Field."
For example, if this sign were placed in an overrun area or access road adjacent to a taxiway it is possible an aircraft that lands long or gets confused taxiing during low visibility might end up there.
In such a case a ground controller probably wouldn't want the airplane to try to figure out how to get back to a controlled movement area on their own, but would want to send a follow-me truck or give them clear instructions how to return to the field.