You might begin by explaining that the phrase creatio ex nihilo points to the belief among many theists that God created the universe ‘out of nothing’. This means that, before the act of creation, there was nothing, no material, from which to form a universe. You might also explain that this is not a view found in the book of Genesis, though others may explain that it may be inferred from it.
You could go on to point out that the idea came about as a reaction to a form of Gnosticism that saw all matter and material as evil. There was a great deal of philosophical debate around these ideas during the second and third centuries. A clearer statement was put forward by Augustine of Hippo in the late fourth century. He argued that, since God alone is Being, he was able to will to exist what had not existed formerly. Some would argue that this did not become the formal teaching of the Christian Church until the fourth of the Lateran Councils. This council stated:
We firmly believe and openly confess that there is only one true God, eternal and immense, omnipotent, unchangeable, incomprehensible, and ineffable…Creator of all things invisible and visible, spiritual and corporeal, who from the beginning of time and by His omnipotent power made from nothing creatures both spiritual and corporeal, angelic, namely, and mundane, and then human, as it were, common, composed of spirit and body.
You may come down on either side of this debate or neither. For example you might argue that, since this creative act included free will, a situation is brought about where judgement is a just thing both in terms of reward and in terms of punishment. Alternatively, you may want to use arguments, such as those of determinism, to say that no blame or praise may be attached to human actions.