Here you can define which programs can, and cannot, run other applications that have access to the Internet or can connect with other computers on your network. This is useful as it can prevent an incoming connection from simply launching another application, which may then be used to perform unauthorized actions. This is a trick commonly used by hackers to collect personal or sensitive information without the user’s knowledge.
If a program is not listed in either category, avast! will decide automatically whether or not the process should be allowed by carrying out various checks, for example whether the program that is trying to launch another application is a recognized program and one that would normally be expected to behave in this way.