As the plot and its subplots carry the characters along, they generally move in a direct, chronological line to the story's conclusion. But some authors like to play with time by introducing flashbacks or through a nonlinear plot line.
What is a flashback?
A flashback (usually presented as a sudden memory) is a literary device that reveals an event that happened earlier in the story. It can be an effective way to give readers information that helps them better understand a character or a plot point, though many editors will advise against this technique because it is somewhat cliché.
A nonlinear plot is simply a plot that is presented to the reader out of chronological order. That is, the story may jump back and forth in time to relate characters to events in the past or future, or it may even begin with the end. For example, readers are greeted with a dying man, and then shown the course of events that brought him to his end. Linear and non-linear plots are both excellent formats, but the nonlinear plot needs extra attention to make the time or character jumps work.
When working with a nonlinear plot, write out the sequence of events in chronological order. No matter how broken up chronologically your novel may be, the story that you are writing is still embedded in time. Keeping the progression of events clear in your head will help you get your story across to the readers.