Complex sentence: Captain Gumbo is a Dutch band formed in 1987, which plays mostly zydeco and Cajun music; that is, music in the French traditions of the U.S. state of Louisiana, based around the accordion.
Split into simpler sentences: Captain Gumbo is a Dutch band formed in 1987. They play mostly zydeco and Cajun music: that is, music in the French traditions of the U.S. state of Louisiana, based around the accordion.