It's a description of Zero-Handoff in Ubiquiti's 1st-gen APs. Fast Roaming is 802.11r.
Getting good roaming is theoretically not that difficult. Lower transmit power so that the device's RSSI in the locations where it's expected to transition to another AP (or off WiFi) are lower than the device's roam scanning threshold (-70 for iOS). Set the minRSSI on APs to something sensible and enable strict mode so that more troublesome devices aren't able to cling to an AP @ -85.
In practice, tweaking those knobs and figuring out placement and channel planning start to get tough beyond 3 APs.
That type of roaming actually ends up breaking a lot of WiFi implementations — it’s really only super useful for WiFi VoIP phones. I worked with a vendor (Bandspeed) doing that kind of WiFi roaming back in 2005, so it’s not remotely a new concept.
Most other devices will jump to another AP broadcasting on the same SSID if the signal is a lot stronger. It’s not nearly as much of an issue as it used to be, but people expect WiFi to Just Work (tm) so it’s better to let the OS’ network stack manage it.
My understanding is that "fast roaming" and many of the proprietary tricks are just ways to speed up the cryptographic pairing process when you switch AP's. I think the same thing happens, but some steps are streamlined.
Edit: meant “zero handoff” per replies below.