Those who felt there would always be time to make the decision. Those who have not yet heard the gospel message given. I'm not going to give you the whole content of the book, but if you want a preview, just read the book of Revelations. It's all in there. Mr. LaHaye and Mr. Jenkins do a wonderful job of making this information easily accessible.
Revelations is full of symbolism.
Some of it is not readily understood. Some of it probably won't be understood until we meet Jesus. Until then, however, this series is one of many that will help us to understand and help us to minister to those who need to hear this message so they won't be Left Behind. My one caution is to remember that this is a fictional series, although Biblically based, and not to use it in place of your Bible. Just like a commentary or any other tool, it can be used to enhance your study.