Core answer
Start here. This is the direct spoken answer to practice first.
Overview
Explains serialization, import safety, start methods, worker failure, and data-transfer cost.
Process workers have separate memory, so submitted callables, arguments, and results generally need to be picklable. Local functions, open connections, locks, and request objects often cannot cross that boundary safely. Spawn and forkserver workers import application modules, so process creation must be protected by an import-safe entry point. Worker crashes surface as pool failures and must be observed rather than retried blindly.