English: “Congregation of Israel” — this is the Sanhedrin; “a matter is hidden from the eyes of the assembly” — they erred in ruling that one of the kares-level prohibitions is permitted, and the public acted on their ruling.
In Sifra / Horayos: The communal chatas is not for a spontaneous mass error but for an authoritative wrong ruling by the Great Sanhedrin that the people then follow. Liability attaches when the hidden matter becomes known.
Type of Learning
Custom (identification of “edah” + conditions of liability).
What Is Learned
The bull of 4:13–21 is brought when the Sanhedrin issues an erroneous permissive ruling on a kares prohibition and the public acts on it.