The first chapter of Genesis (which is the first book of the Bible) tells the story of God creating the world in six days. On the seventh day, He rested. Some people believe that Creation literally took six calendar days, of twenty-four hours each.
Some creationist scientists go so far as to say that this is plausible due to Einstein’s Theory of Relativity. Many people know, from high school science class or from watching Star Trek, that time slows down near a black hole. Since the gravitational pull is so enormous, not only is light unable to escape its clutch, but time itself is affected. If that is true, imagine how slow time would be at the epicenter of the Big Bang.
At one moment in the early history and birth of the universe, all known matter was compacted into one very small and tight space. The density was enormous yet the size was small. Time near the center of the Big Bang would pass by more slowly than in the outer regions farther away from the center. So it is theoretically feasible that a literal day near or at the center of the Big Bang would appear to someone galaxies away as millions of years.
Another (and more plausible) explanation is that Genesis is not a science book. It is the revealed, inspired, and inerrant written word of God, yes; but it was not written by scientists or historians. God inspired human beings to write, but He did not choose scientists or historians. He chose men of faith to explain things of faith.
The purpose of Genesis is to teach the truth that God is the Creator. The details of telling that truth are meant to be taken in context. As we will see in another question on how to interpret the Bible, Catholic Christianity warns its members never to take a Scripture text out of context, lest they get a pretext.
Figures of speech are not considered lies or deception, but neither can they be interpreted literally. If I say someone is pulling my leg, I mean he is joking with me, not that he is yanking on one of my limbs. Similarly, the Bible uses many forms of literature and literary genre, and some of those are metaphor, analogy, hyperbole, and other figures of speech.
So when the Bible says God created the earth in one day, it may just be a figure of speech; the intended meaning and faithful interpretation is that God created in a sequential order, or that He works systematically, not chaotically.