As Sashimister has already mentioned, this 2 lines are quoted from a Chinese poet named Li Bai (around the 7th century). He went to a place called Lu Shan and wrote this poem to express his astonishment at the beautiful scene of the waterfalls.
飛流直下三千尺
飛流直下 refers to the motion of the waterfalls
飛流=flying and flowing; 直下=straightly down into
三千尺 = 3000 feet
This sentence describes the rapid drop in elevation of the water flow
疑是銀河落九天
疑是 as if
銀河 the Milky way
落 drops/falls into
九天 the Heaven (an ancient Chinese word for Heaven)
This sentence could be translated into something like "As if the Milky way falling from the Heaver high above"
Hope it helps.
