A xylophone is a kind of musical instrument. You make music by hitting the different-sized keys with small hammers to play a tune.
The keys on xylophones are typically made out of wood, although some modern versions are made from plastic or fibreglass. Very early xylophones had keys made from gourds. Xylophones are very similar to glockenspiels and vibraphones. Glockenspiels are made from metal and have a higher pitch, while a vibraphone is also made of metal but is a more modern instrument and has a sustain pedal. The instrument in the picture is actually a vibraphone.
Most people call all of these similar instruments xylophones. The most popular image of a "xylophone" is the children's six or eight key version with brightly colored metal keys commonly seen in toy stores.