I have read the words Banī Isma’īl so many times in the Qur’ān but I don’t know who or what the Banī Isma’īl are.

The word ‘Banī’ or ‘Banū’ means ‘children’. So the expression implies the children of the Prophet Isma’īl (sws). Historically, we know that they were the major inhabitants of Arabia during and before the time of the Prophet (sws). They had become involved in polytheism (shirk) and openly said that this was the religion Allah had revealed; precisely because of this belief, they were also called ‘Mushrikīn’ (pagans; idolaters). The Old Testament uses the word ‘Ismaelites’ for them, just as it uses the word ‘Israelites’ (Banī Isra’īl) for the children of the Prophet Isaac (sws), who we know was a brother of the Prophet Isma’īl (sws).