Compromise assessment involves proactively searching for malware and attackers on your network which may be present for some time. They can silently exfiltrate data, listen patiently to confidential data or scan the network to get credentials strong enough to steal key information.
You should be able to stop majority of the threats by keeping the basic hygiene of your network security by properly implemented Endpoint Security solutions, network security devices, and other security tools. However, once an attacker has sneaked into your network, without getting detected, there is nothing much you can do to stop them.
