A .so file is a Linux shared object: a compiled library that the operating system can load at runtime. In Python, CPython can also import certain .so files as extension modules, so they behave like ordinary modules even though they are binary code, not readable .py source.
This matters in cyber security because compiled modules are harder to inspect quickly and may evade simple string-based detection. Attackers can package malicious logic as a .so to look like normal build output, hide implementation details, and force defenders to rely on behavior rather than source review. Defenders should watch for unexpected .so loads, binary modules in user-writable paths, unusual build activity, and suspicious process or network behavior at runtime.