As the song plays, the measure being played highlights so you can keep your place. You can choose its color, or hide it altogether.

 

On Mac, click the chord chart settings button (Aa) and choose Playback Position, then a color, or Hidden. On Windows, open Settings from the cog at the top right, or File > Settings.

 

Hiding the marker is a good way to test whether you really know where you are in the form.

 

For the full guide, see Play a song on the iReal Pro website.