Very short post, but there was nothing easily available online to explain this so I had to figure it out myself. Hopefully this will help others.
On the Pi 3B+ there is the usual RJ45 socket with two LED's mounted at the base, one green on the left and one orange on the right.
On previous Pi computers and most other systems ever made, the green light is constantly on to convey that the network device is 'up' and operational. The orange light is then always on but flashes when there is activity on the line.
The Pi 3B+ now has a pseudo-gigabit network capability. The interface is a gigabit port but runs over a USB2 Bus so throughput is actually approx 300Mb/s rather than 1000Mb/s, but it's still a 200% increase on the 100Mb/s of the previous models.
To help diagnose network problems and/or tell users what speed the network attached to the Pi is running at, the Raspberry Pi devs have used the lights to signify whether 10/100Mb or 1000Mb is being used by the interface. If a 10/100Mb network is connected then the green light will illuminate and then flash when there is network activity. The orange light will remain off. When there is a Gigabit network attached, the green light stays off but the orange one lights up and flashes.
Wish that had been written down somewhere.