Definition
Wi-Fi roaming is a networking technology that, as you move with a smartphone or laptop, hands your internet connection from a wireless access point (AP) with a weakening signal to another AP with a stronger one in a split second, so you barely notice any interruption.
The secret to walking around on a video call without dropping
Picture a relay runner passing the baton to the next runner at full speed, without slowing down. Walking through a big office or a huge shopping mall while watching YouTube or on a video call, and barely seeing a stutter, works in a similar way.
One router can't cover every corner of a large space. So buildings usually install many wireless access points (APs, the devices most people call Wi-Fi routers) at regular, close intervals. When you walk down a hallway with your phone, you move away from the first router you were connected to, and its signal strength starts to fade.
At that point, your phone quickly scans the nearby airwaves and finds a much stronger router close by. Then, in a moment too short for you to blink, the connection baton is smoothly passed to the next router. One difference from a real relay race: in a relay, both runners hold the baton for a moment, but a device is usually connected to only one router at a time. So there's a tiny gap while it lets go of the old connection and attaches to the new one. When conditions are good, that gap is so short you barely feel it. This whole process is called Wi-Fi roaming.
How is it different from a regular router?
In a Wi-Fi setup without roaming support, hopping from one router to another is slow and frustrating. Your device decides when to switch, and many devices cling to the router they first connected to until its signal gets very weak, even if a better one is right next to them. This is often called the "sticky client" problem.
For example, say you walk from the living room into a bedroom that has its own router turned on right beside you. Your phone still hangs on to the living room router far away. Eventually the internet cuts out, and only then does your phone fumble its way onto the new router's signal. In that moment, a live online game kicks you out and a streaming video freezes.
In a smart roaming setup, on the other hand, once the current router's signal starts dropping below a certain level, your device checks the status of nearby routers ahead of time. So apart from the very brief moment of the switch, it glides over to the new router with the better signal, and you barely notice any interruption.
A little more precisely
To be more precise, truly smooth Wi-Fi roaming needs more than good routers. Your devices, like smartphones and laptops, must also support the related international Wi-Fi standards. Most notably, '802.11k' and '802.11v' help your device pick the next router to move to quickly, and '802.11r' cuts the re-authentication time when you move.
Without fast roaming, every time you move to a new router, your device has to re-check the password keys and authenticate from scratch. In business Wi-Fi that uses an authentication server, like at a company or school, this re-authentication can take hundreds of milliseconds or more. Fast roaming technologies like 802.11r share the security credentials you already passed on the previous router with the next one, greatly shortening re-authentication time.
These days, Mesh Wi-Fi systems, which tie several devices together under a single Wi-Fi name (SSID) in homes and cafes, have become popular, which makes it fairly easy to set up a roaming environment at home. Just keep in mind that how smooth it actually feels depends on whether your devices support the standards and on the product's settings.
Common misconceptions
Wi-Fi roaming is the same thing as the "data roaming" you sign up for with your carrier when traveling abroad.
The names match, but they're completely different things. Carrier data roaming is a paid service that lets you borrow a foreign cellular network, while Wi-Fi roaming is a wireless networking technology that links multiple routers together so you barely notice the handoff.
Where you meet it
Walking down the hallway of a big hospital or university while on a voice call over a messaging app, and the call doesn't drop along the way.
Installing a mesh Wi-Fi system in a two-story house so that a high-definition video keeps playing without pausing as you climb the stairs from the first floor to the second.
In one sentence
Wi-Fi roaming is the technology that, as you move, hands your connection from a router with a fading signal to a nearby one so smoothly that you barely notice any interruption.