It's a setting that allows you to flip the positions of the oncoming notes horizontally in the rhythm game.
The setting can be applied to all shows.
There are no special bonuses or rewards for playing with Mirrored Lanes. You will earn the same scores and Event Points as you would without.
This also applies to your results even if you achieve All Perfect or a Full Combo.
Furthermore, in Co-op Shows and such, your matched opponents will not be notified if you are using Mirrored Lanes.
Tap ON to enable Mirrored Lanes through Menu > Options > Show > Mirrored Lanes. Tap OFF to disable.