Theologia Crucis or Theology of the Cross means looking to the Cross as the centre of all Christian theology & life. Martin Luther contrasted the Theology of the Cross with Theologies of Glory. According to theologies of glory, man can reach God's righteousness or revelation himself. This doesn't mean that a theology of glory necessarily denies Christ's atoning work at the Cross; however a theology of glory will look to Christ's work as something that helps man on his way. An example of this could be in seeing the gospel simply as the way to become a Christian, and then looking to other things in order to make progress in the Christian life. According to the Theology of the Cross, on the other hand, it is only through the gospel that we can be sanctified. Becoming a Christian and living as a Christian both rest on God's grace at the Cross.
The theologian of the Cross knows that God reveals Himself most graciously & clearly, not through reason, nor even through outward blessings and spiritual experiences, but, rather, through the Cross of Jesus Christ.
The Theology of Glory says that man is capable. The Theology of the Cross says that man is completely incapable. The theologian of glory attempts to move on beyond the Cross. The theologian of the Cross knows that, being completely incapable in ourselves, we can never go beyond the Cross.
For more about being a theologian of the Cross, I recommend Gerhard Forde's book, On Being a Theologian of the Cross: Reflections on Luther's Heidelberg Disputation, 1518 (Grand Rapids: Eerdmans, 1997).
You could also read this article on Luther's Theology of the Cross by Carl Trueman.