I want to setup wireless roaming, and on the openWRT wiki, it looks relatively straightforward, but I'm not real clear on this "Mobility Domain" thing?
To configure using the Luci web interface, go to Network→Wireless, and on each SSID
click Edit, scroll-down to the Interface Configuration tabs, select WLAN Roaming,
check the 802.11r Fast Transition box, and put in a 4-digit Mobility Domain that
matches the same SSID on your other 802.11r-enabled routers/AP's.
Is this 4-digit number something I can just pick, and it just needs to be the same on all of the Wireless AP's that I want to roam across?
mobility domain is a random value that tells clients that this other AP (same name + subnet) is a friend, and they can jump there with quick re-authentication and skip dhcp. In practice it matters only for voip phones and eap.
Note that 802.11r is only really a significant win if you are using systems which are slow to authenticate. In practise this means 802.1X "enterprise" authentication (where each user has their own username and password to authenticate to wifi instead of a single passphrase "PSK" for the network SSID).
That's because in these configurations, typically the authentication server is remote from the access point, and in some systems might be in a different country entirely with a high round trip between the wifi access point and the authentication server.
Also note that some older buggy clients will refuse to connect to the network at all if 802.11r is enabled (although thankfully most of those have now been fixed or scrapped), and that many others won't use it, or will only use it when 802.1X is in use. Because of this I only enable 802.11r if I know that I'm really going to need it.