Mawali is the name for people all over the Islamic Empire who were not Arab but had converted to Islam. According to the Koran, anyone who converted to Islam was supposed to be considered the same as anyone else. All Muslims were sisters and brothers, and Arabs weren't better than Syrians or Egyptians or Persians. Black or white, skin color also made no difference. And nobody who followed Islam, even a mawali, was supposed to pay any taxes.
But under the Umayyad dynasty, the Arabs didn't like this. They had racist ideas that the only good people were Arabs. They didn't like the mawalis.
It was partly because of this that the mawalis rebelled against the Umayyad rulers and put in a new dynasty, the Abbasids, that promised to treat the mawalis fairly, in accordance with Islam.