What does 2x bass pedal mean in Rock Band?

“2x Bass Pedal” means a song is designed for players using two bass drum pedals instead of one.

Some drummers use two pedals to play faster or more complex drum patterns, especially in metal songs. Songs labeled (2x Bass Pedal) include drum patterns that match this setup and are intended for players who want a more accurate experience.

Standard versions of songs are adjusted so they can be played with a single pedal.

Note: This feature applies only to legacy Rock Band titles. It is not supported in Rock Band 4.