>What is it
It looks like a "headphone amp", anon. Do you play electric guitar?
Apparently it's a device into which you plug in the wire from the pickup of an electric guitar (your amp seems to be built for bass guitars) so it could produce sound. You'll have to use a speaker or a pair of headphones with this amplifier, that's why it's a "headphone amp", as opposed to an amp with a built-in speaker.
$5 is a very low price for this, on the Internet they seem to go for about 80 dollars. Good find!