company secret; confidential (outside the company)
Used in corporate contexts to label information that must not be disclosed to outsiders. Often seen on documents or in meeting materials.
This document is confidential, so please do not take it outside the company.
社外秘の情報をうっかり話してしまった。
I accidentally disclosed confidential company information.
内部 means 'internal' or 'inside', and is used in phrases like 内部資料 (internal documents), but 社外秘 explicitly marks information as not to be shared outside the company.
Compound of 社 (company), 外 (outside), and 秘 (secret). The reading is on'yomi throughout.