Compatible with iOS 7 and iOS 8.
It's an equalizer. That applies everywhere.
To be specific, it applies an equalizer to your speaker output. So it works with every app, hence EqualizerEverywhere. You can configure it from the control center (by swiping left on the music controls) or through settings.
For the nerds:
10 bands with a preamp. Peaking EQ filter. 60Hz, 170Hz, 310Hz, 600Hz, 1KHz, 3KHz, 6KHz, 12KHz, 14KHz and 16KHz. Ranges from -12dB to +12dB.
Use the equalizer either through settings or in the control center.
See larger Screenshots below..