She was a Greek woman who was cursed by Apollo to know and foretell true prophecies which no one would believe. She would often predict calamities (like the fall of Troy) that were disregarded.
The name is sometimes used to refer to someone who predicts doom or future problems (though whether or not they are true or believed seems not to matter).
The actual name “Cassandra” is from the Greek word for “helper of men.” It has nothing to do with the reputation the name was given in Greek mythology.
Why I Looked It Up
In a book about the Union Carbide disaster in Bhopal, a newspaper reporter who warned the public (quite correctly, it turned out) about the dangers of the chemical plant was referred to as a “Cassandra” several times.