Bilal was a slave in Makkah. When he heard about Islam, he immediately saw the beauty and wisdom of its teachings. He decided that he was going to follow Islam and become a Muslim.
This was a big step for him because his master would not approve of this. The Makkans were against anyone who became a Muslim. Islam taught that worshipping idols was wrong and the Makkans were idol worshippers.
When Bilal's master found out about him becoming a Muslim, he became mad with rage and anger. He punished Bilal very severely and ordered him to leave Islam.
However, Bilal could not leave the truth and instead remained a Muslim. This caused his master to torture him so bad that even the people around him were horrified by it. Bilal still persisted and remained a Muslim.
The Makkans would take turns at night to whip him and during the day, they would throw him on the burning-hot sand and put a heavy boulder on his chest. Bilal suffered so much but the only words which came from his tongue would be 'Ahad! Ahad!' - Allah is one! Allah is one!
Finally, the Muslims managed to free Bilal and the suffering eased for a while. Bilal became one of the most respected and best loved Muslims. He was given the honour of being the Muazin of the Prophet (pbuh) and would call all the Muslims to prayer.
Bilal wasn't an Arab but a black Abysinian slave. None of this mattered to the Muslims and even today, they continue to love and respect him. He is always spoken of in the highest terms.