A Bible commentary is a separate, stand-alone resource that attempts to explain the meaning of the Bible so the reader has a clearer understanding though a study Bible has these comments adjacent to the text they relate to. The nature of these written notes can vary in their intent. Some are devotional having been designed to assist in applying the Bible to daily life, while more academic commentaries often refer back to the original Hebrew and Greek texts to extract the meaning.
Any such humanly created material will reflect the beliefs and perspective of the writers and so should not replace our own personal study of the Bible and Spirit-led exploring, however they can give additional beneficial insight. Because the Bible reflects God’s instructions, it must take precedent over any human opinion or understanding.