The short answer is "it's a metal xylophone." 

But... it is more involved than that...

The vibraphone is a tuned percussion instrument. The typical vibraphone has three octaves. The "bars" that the performer strikes are arranged in the same way as the keys on a piano, with "black" keys on the top, and "white" keys on the bottom (but, on the vibraphone, they are all the same color). So, if you can play the piano, you can definitely play the vibraphone.

Because the bars are made of metal, they can "ring" or "sustain" for a long time, which is why vibraphones have a sustain pedal, also similar to the piano. 

But the "vibra" part?

Vibraphones have pipes under each bar, which allow the sound of each bar to "resonate," similar to how strings are allowed to resonate through the hole in an acoustic guitar. Covering each resonator is a fan blade, that, when connected to the electric motor, gives a vibrating "soft/loud" or "wa-wa" effect.  

ERIC KATZ is a JAZZ instrument?

Yes!!! The sound has been found to fit perfectly with the sound of other typical jazz instruments...saxophone, trumpet, trombone, piano, bass and drums. 

‚ÄčAnd unlike most other instruments, the first style of music played on the vibraphone was jazz. The legend goes that in 1930, drummer LIONEL HAMPTON was in NBC studios for a session, and stepped up to the vibraphone, which had been built to create sound effects (remember the G - E- C tune for NBC?). He started to try it out, and soon brought it to a recording session with Louis Armstrong. Their "Memories of You" is the first known recording of the vibraphone in a jazz setting.

What is the Vibraphone?

Oh...and one more thing...

Cool people call it the "vibes."