It is the central idea behind your story. A theme is a natural, unobtrusive part of a story. This is the theme, the quality that brings with it a sense of values and drama. People sometimes confuse plot with theme. The plot is the series of events that proves or disproves the theme.
Plot is a series of conflicts, literal happenings.
Common conflicts are man against man, man against nature, and man against himself. A theme is often not profound. It is usually simple and appeals to our emotion and intellect. A small slight has huge repercussions Man is helpless against nature, society, women, etc. The theme statement keeps the author focused.