Sandboxing
Opening a suspicious file in an isolated environment to watch what it does before letting it near a real system.
Email security products use this to detonate attachments: the file is opened in a disposable virtual machine and its behavior observed. If it drops a payload or calls out to a suspicious host, the message is blocked.
Attackers respond with evasion, adding delays, checking for virtual machine artifacts, or requiring a human interaction before the malicious behavior triggers. It raises the bar rather than closing the door.
Where this comes up
Sandboxing sits inside our cybersecurity and threats work. If you are trying to work out what this means for your own environment rather than in the abstract, that is the page worth reading next, and a short call will get you a straight answer faster than either.