A one-volume commentary contains a concise commentary on the books of the Bible in a single comprehensive reference, often accompanied by a variety of essays and critical insights on Biblical scholarship and interpretation.
Different commentaries (whether a series or an individual volume) often represent different perspectives; some will be broad, some will have a very specialized focus. Read commentaries to get a variety of theological, scholarly, and cultural perspectives. While you may come to favor one commentary over others, be open to encountering other voices.