Deep packet inspection
Also known as DPI, SSL inspection
Examining the contents of network traffic, including decrypting HTTPS, rather than just the addressing information.
Since almost all traffic is now encrypted, a firewall that cannot decrypt sees only destinations. Inspection means the firewall terminates the encrypted session, examines the content, and re-encrypts it onward.
It requires deploying the firewall's certificate to every device, breaks applications that use certificate pinning, and carries privacy considerations. Most deployments inspect selectively rather than universally.
Where this comes up
Deep packet inspection sits inside our networks and infrastructure 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.