The host process is the main application that runs a sandboxed component inside it. In practice, it owns the operating system resources, native libraries, memory, and privileges that the sandboxed code can indirectly reach. The sandbox may restrict what guest code is allowed to do, but it still depends on the host process to enforce those limits.
This matters because a flaw in the sandbox layer can affect the entire program, not just the isolated component. If untrusted JavaScript, plugins, or extensions trigger a memory-safety bug, the result may be a crash of the host process or, in worse cases, control-flow hijacking inside that process. Defenders treat the host process as part of the attack surface: keep it patched, minimize privileges, avoid exposing sensitive objects to sandboxed code, and assume that in-process isolation is weaker than a separate process boundary.