“Bella ciao” is an Italian protest folk song that originated in the hardships of the mondina women, the paddy field workers in the late 19th century who sang it to protest against harsh working conditions in the paddy fields of North Italy.
Taken together, ciao bella is a colloquial, familiar way of saying “hello” or “goodbye” to one woman (as opposed to a group). The masculine version of the expression is ciao bello.