What does Romans 16:6 mean?

Paul continues the list of people he would like his readers in Rome to greet for him, mentioning most by name and at least one commendable quality. Next, he asks that they greet a woman named Mary, or possibly "Mariam." As seen in other passages in the New Testament, the name "Mary" was so common that it was not unusual to see several women in the same group bearing it (Luke 24:10; Matthew 28:1) Her very common name indicates she may have been a Jewish woman, though "Mary" was occasionally used by Gentiles, as well.

Paul praises Mary for working very hard for the church in Rome. In fact, she "toiled greatly." We don't know what this work consisted of or how Paul would have known her, but she is praised for her diligence in serving the Christians there.