The way most people understand the terms “Son of God” and “Son of Man” today is probably at odds with how they would have been understand by many Jews in the first century. In our way of thinking, a “son of God” would be a god (or God) and a “son of man” would be a man. Thus, “Son of God” refers to Jesus’ divinity and “Son of Man” to his humanity. But this is just the opposite of what the terms meant for many first-century Jews, for whom “son of god” commonly referred to a human (e.g., King Solomon; 2 Samuel 7:14) and “son of man” to someone divine (cf. Daniel 7:13-14).
In the New Testament gospels, Jesus uses the term “son of man” in three different ways. On some occasions, he uses it simply as a circumlocution for himself; that is, rather than referring directly to himself, Jesus sometimes speaks obliquely of the “son of man” (e.g. Matthew 8:20). In a related way, he sometimes uses it to speak of his impending suffering (Mark 8:31). Finally, he occasionally uses the term with reference to a cosmic figure who is coming to bring the judgement of God at the end of time (Mark 8:38), a judgement that Mark’s gospel expects to be imminent (Mark 9:1, 13:30). For Mark himself, of course, the passages that speak of the coming Son of Man refer to Jesus, the one who is returning soon as judge of the earth. As we shall see later, scholars debate which, if any, of these three uses of the term can be ascribed to the historical Jesus.
The Bible: A Historical and Literary Introduction by renowned biblical scholar and New York Times bestselling author Dr Bart D. Ehrman, p. 266.