This is a common question, and even after five years of being certified in the program, I have to admit I sometimes struggle to fit into a concise sentence, because on the one hand, it is so very very simple and on the other so very difficult.
What is RDI? Applying typical development to kiddos for whom this has been a challenge.
RDI Parents are often pleasantly surprised to find that there are no secret strategies to employ. It's all the things parents are naturally amazing at, reading their child, providing safe challenging opportunities for growth, spotlighting memories and transferring wisdom. The trick is doing all these amazing things at a slow, deliberate and individualized pace for their child on the spectrum.
It also seems most helpful to be able to "see" RDI. Amy Cameron and a wonderful family of hers have provided a window into the world of an RDI family.
A quick search of RDI on youtube has many more examples by brave families who have put their learning out there for others to see.