to speak candidly; to be frank and open about one's true feelings (idiomatic).
長年の友人と夕食をしながら腹を割って話した。
I had dinner with a long-time friend and we spoke frankly with each other.
問題を早く解決するには、関係者が腹を割って話し合う必要がある。
To resolve the issue quickly, the parties involved need to have an open and frank discussion.
まずは腹を割って本音を聞かせてください。
First, please be candid and tell me your true feelings.