This is a Realistic 3 Band Stereo Parametric Equalizer unit. These were sold at Radio Shack in the 1980s. There are left and right RCA input and output jacks. There are sweepable frequency bands for bass, midrange and treble. You can boost or cut up to 12 dB for each band. There's also a master volume.
The unit is in good condition. You can power this with six AA batteries, or with a 9-volt power supply with a 2.1 mm negative center (like a typical pedal power supply).