Only first-generation AirPlay receivers, an AirPort Express and its contemporaries, need this. They hold a fixed two-second buffer, so the audio arrives two seconds behind the position marker on your chart.

 

On Mac, open Preferences > General and switch on AirPlay 1 Offset. It moves the marker to meet the sound.

 

Leave it off for anything else, and off is the default. AirPlay 2 receivers (HomePod, Apple TV, modern third-party speakers) carry timing information with the audio and stay in step on their own. There is no equivalent setting on the other platforms.

 

For the full guide, see Sound output and audio devices on the iReal Pro website.