Application Whitelisting, means a machine or device will likely only run software it is approved to run. It will not likely not run any other software. That is, the virus or malware *may not be able to run.
It can be however bypassed with some effort e.g. by exploiting the software that is running the whitelisting process, among other ways. It is important, like any software to patch application whitelisting software. That is, to keep the application whitelisting software up-to-date.
Application Whitelisting is one of the ASD’s Top 4 Mitigation Strategies.
Application whitelisting is a good thing to do, but it is not bullet proof.