WiFi roaming is highly desired on WLANs (WiFi) today, and have been that for many years now.
Roaming originates from the mobile world. The term roaming means, being able to move from a "Home network" to a "Visited network" and still have voice and data connection. We still use this term in the mobile world. We can use the same reference in WLAN. Instead of "Home network" and "Visited network", we have AP1 WiFi_1 and AP2 WiFi_1.
BSSID
When you, as a user, connect to the WLAN on a specific SSID, you connect to any AP broadcasting this SSID (ie. 'Guest', 'Private' & 'Setup'). SSIDs keeps the traffic within the WLAN. Since data in a network always need a destination, there has to be a way to identify the APs and their associated clients. This is the job of the BSSID (Basic Service Set Identifier).
BSSID is the MAC address of the radio on actual AP for the specific band. On TRIAX EoC you will see a BSSID for every SSID being broadcasted from the AP, on each band. So - If you are broadcasting 8 different SSIDs from the same AP, you will see 8 different SSIDs with the same BSSID, for each band.
Roaming
Being stationary is not a part of the "WiFi idea"! People will move around and connect to the WLAN from different locations and will be moving around. The client connected to a SSID wishes to stay connected but WLAN has a limited range. Therefore will the client will try to reconnect or re-associate to the WLAN. When you come home from work, your smartphone will automatically reconnect to your WLAN, as soon as it is possible for the smartphone to do so. Almost the same (a crude comparison) happens when you leave the range of 1 endpoint's WLAN and enters the range of the next endpoint.
Lets take an example:
1. You are located in the last room on a hotel floor and are connected to the SSID "Guest". You are on your way down to the restaurant.
2. As you leave the room, your smartphone loses the connection the Endpoint in the room. The smartphone will automatically re-associate to another endpoint (on a new BSSID), broadcasting "Guest", before the connection is dropped. You will only experience the re-association, if you are using the WLAN in that short time frame (seconds). This is done without using the amendments -k/-v!
3. This process of re-association will occur every time your smartphone loses the connection but can see a new BSSID broadcasting the same SSID (in this example being "Guest").
4. Eventually you get to the restaurant and properly re-associate to a new BSSID broadcasting "Guest"
Bon Appetit :-)