It means to jump or dance. A lot of words often pair two similar or synonymous words together, oftentimes for emphasis. A lot of times they will be a Northern and Southern word combined. For instance there's chăn mền where chăn is Northern and mền is Southern, individually or together they mean blanket.
When you say "nhảy múa" you refer to a general kind of dancing, not a specific tipe, if the subject was dancing hip hop then you'd say "Họ múa hip hop".