Ableton Link synchronizes beat, tempo, and phase across multiple apps and devices over a local network, and this short walkthrough from Ableton's Learn Live series covers what it is and how to switch it on in Live. To activate it, go to Preferences, open the Link MIDI tab, and click Show/Hide Link. A Link button then appears in the top-left corner of Live's interface.
Once enabled, Link lets you connect iOS apps or hardware running on the same network, keeping everything locked to a shared tempo in real time.