Here's the description from the mxr page.
MXR M-103 BLUE BOX
This unassuming little box shovels out enough low frequencies to scare a blue whale. It takes your guitar signal, fuzzes it up, then duplicates it two octaves down. You control the output and the mix between the dry signal and the effect. Using it 100% wet is quite an experience, but more recognizable sounds appear as you dial In more dry signal. The Blue Box is known for having a chaotic personality that can make each session a totally unique experience!