The Septuagint (often referred to as LXX) is a Greek translation of the Old Testament completed prior to the time of Christ. While the Old Testament was originally written largely in Hebrew, the New Testament was written largely in Greek. Furthermore, it appears that when the New Testament quotes the Old Testament, it is often the Septuagint version that is being quoted.
Here’s a better, though still brief, rationale for why the Septuagint is important from Dr. William Varner and his DrIBEX ideas blog. (Yesterday was International Septuagint Day.)