Thematic threads hold a story together. Beyond the events or characters, theme transmits meaning.
Theme is different than subject. You may want to write a book about _______ (fill in the blank), but that's your topic, your exploration. From it, if you're lucky and paying attention, theme will emerge organically. Creating that tug at the reader's mind and heart that doesn't go away when the book is over.
Trying to write about a certain concept--esoteric sadness, for instance--won't necessarily result in the reader taking that theme away from your book. Poorly done, it will feel like you are telling us your opinion, or sharing great thinkers' opinions.
Theme works best when there's a surprising undercurrent of meaning that threads through important moments in the story.
Next week, I'm teaching a new eight-week online class on theme and voice. I'm combing through my own studies on theme. I've collected some great exercises and what I call "lures" for theme, that help you catch your writing's theme (as if you're fishing and waiting for the bite). Although the class goes into a lot more depth than I can give in this post, here are three "lures" that are fun to work with--and very helpful for catching your theme.
Still room in my online class on voice and theme.