Ableton Link is an open source protocol that helps musicians keep in time. It's primarily used to sync the tempo of different devices or software.
It's design is very much like an open jam session. Once it's running, everyone can join the Link session and control it. It works over network, so anyone connected to the same network will be able to join in.
By default, Link is hidden in the View menu. After making it visible via View > Show Ableton Link, a button will pop up in the toolbar. You can then click the button to turn Link on.
At this point, Resolume will start looking for any other software in a Link session. If it finds any, it will join automatically. It will show you how many other devices are currently in the session.
It will now automatically keep the BPM and the current position in the measure in sync between all connected computers. Change the BPM in Ableton and Resolume will follow. Nudge the BPM forward a bit in Resolume, and Ableton will follow.
When using Link, the Resync and Pause buttons for the BPM are disabled. It's considered bad musical practice to do a hard reset of the measure like this, or completely pause the BPM altogether.
Great power, great responsiblity
Link assumes complete equality of everyone in the session. This means you can change the BPM just as abruptly as anybody else. So when you hit x2 on the BPM to accentuate a musical climax, the BPM on all other connected softwares will speed up as well.
This is great if you're just using Link to keep multiple VJ computers in sync. But your DJ buddy might not be so amused by it.