Consider the question "What is a Lutheran?" Here are three wrong answers. Lutherans worship Martin Luther. (No, we worship the Triune God.) Lutherans follow and believe every word that Martin Luther ever said or wrote. (No, we believe certain writings are correct because they agree with the Bible.) Lutherans just didn't want to be Roman Catholics anymore. (No, the early Lutherans sought to reform teachings and practices in the Roman Catholic church which they believed to be in error based on the Bible.)
If those are wrong answers, what are some correct ones? Well simply, a Lutheran is a Christian. During the 1500s the Lutheran Reformers wanted the church to believe, teach, and confess what the Bible teaches, and what the apostles taught in the New Testament. Period. They wanted to bring the church back to Biblical teaching and practice from which it had drifted over the centuries. The Reformers had hoped that the pope and other church leaders would see the errors that had crept in to the church's teaching and practices. When they realized that wasn't going to happen, they went out on their own.
Martin Luther never wanted this breakaway group of Christians to be named after him. In fact, the term "Lutheran" was a derogatory term invented by the Roman Catholics! Nevertheless, the name stuck. Churches in the Lutheran Church–Missouri Synod subscribe to the writings in the Book of Concord from 1580, usually referred to as The Confessions. These writings include the Apostles, Nicene, and Athanasian Creeds and writings of the Lutheran reformers like Luther, Phillip Melanchthon, and Martin Chemnitz. Subscribing to The Confessions means that we hold that the doctrine taught in these writings is in accord with and agrees with the Word of God as contained in the Old and New Testaments. Other conservative Lutheran groups are similar though they may not subscribe to all the documents in the Book of Concord.
For more info on what's in the Book of Concord or to read it, check out bookofconcord.org.