Cities in John's day had a city register which listed the names of every citizen. If a person committed crimes, or otherwise defiled his standing in the city, he could be called before a tribunal and his name removed from the city registry, literally blotted out. He would no longer be considered a citizen of that metropolis and would live from then on in anonymity or be forced to move elsewhere.
This concept forms the background for the Book of Life as John describes it. It is a book originally containing the name of every person ever born into this world. If that person dies having rejected God's offer of salvation, his name is blotted out of the Book of Life. It is a sobering thing to think about a person paging through God's Book of Life in vain looking for his name and not finding it. William Newell, a great scholar and commentator on Revelation, said there are four things to be noted about the Book of Life: