This idiom is a reference to the novel 'Alice in Wonderland' and refers to a process or journey that becomes more unpredictable, unexpected and strange as you continue. In the story, Alice literally falls down the hole of the White Rabbit, taking her to Wonderland. You might use this to explain that you are going on a journey into the unknown, or that you expect problems or strange things to happen. This idiom is common in both social and professional situations.
The group of friends fell down the rabbit hole when they began talking about conspiracy theories.
I have never had a client in Japan before. This will be a journey down the rabbit hole for me.