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

 

Tap the chord chart settings button at the top of song view, which reads Aa on iPhone and iPad and A on Android, then tap Playback Position. On iPhone and iPad, choose the color of the marker. On Android, toggle it on or off.

 

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