The way I understand boulders is the things that need to be addressed before talking about the little things, so any tension / important topics that act as a source of confusion between you in your relationship, and that by talking about you can then feel safer and more comfortable in each other’s presence.
You clear away the big boulders, the medium boulders, etc., whittle it down, with the goal of making it comfortable to talk about the small things.
Hey, that’s okay, it’s something I learnt the hard way and am dealing with catch-up at the moment.
In the book How to Win Friends and Influence People, it talks about how you should apologise after some wrongdoing as soon as possible.
This is a similar concept, following this concept the apology is the boulder. It would be uncomfortable talking about nonsense / small things when this hasn’t yet been cleared up.
In the news recently there was this talk about two long-lost twins who found each other on social media for the first time, and they then had to confront their adoptive parents on that big boulder there. For the step-parent, that boulder would have had to have been sitting there for 19 years! Imagine the relief and the safety / comfortability that would arise when that boulder was finally lifted. They could finally talk about the more nuanced things without that weight on their shoulders, that filter. It opened a whole new channel of conversation.