Something that's surpassing is exceptional or outstanding — much better or more than usual. A scene of surpassing hilarity in a movie might make you laugh until tears run down your face.
The adjective surpassing is a bit old fashioned, and you're most likely to read it in a novel or poem. Its most common use is in the phrase "surpassing beauty," or exceptional beauty. A poet might reflect on the surpassing beauty of the sunset over the ocean, for example. To surpass is to "go beyond," from the prefix sur-, "beyond," and the Old French verb passer, "to go by."