This is a reference to Jaʿfar al-Ṣādiq's position on the consequences of ignorance of the law, especially among new converts. Hossein Modarressi cites this reference as an example of one of the legal presumptions of al-Ṣādiq's legal school: the effect of ignorance.
This source is part of the Online Companion to Hossein Modarressi, Text and Interpretation: Imam Jaʿfar al-Ṣādiq and His Legacy in Islamic Law (Harvard University Press, 2022).