A module cache is the local storage area where dynamically loaded Python modules are saved so they can be reused without fetching or rebuilding them again. In Python tooling, this often means files or bytecode are kept on disk after an import or download step.
It matters in security because cached code can persist even when a user later refuses to trust remote code. If an attacker can influence what gets written into the cache, the file may remain on the system and be imported later under a different context. Defenders should treat cache writes as security-sensitive: trust decisions should happen before persistence, and cache contents should be reviewed, pinned, or cleared when remote code is involved.