According to the Bible, “Son of God” means: (1) He is fully God. (2) He is one of three persons who together make up the One God. This concept is called the “Trinity.”
The religious leaders of Jesus’ time understood this concept: This was why the Jews were seeking all the more to kill him, because not only was he breaking the Sabbath, but he was even calling God his own Father, making himself equal with God. (John 5:18 ESV)