The simplest definition of a theme is that it is a "central and dominating idea in a literary work." Usually, themes are related to some enduring or universal idea, a literary trope or motif, or even a debate or conflict that addresses basic aspects of the human condition. For example, important themes in Romeo and Juliet would include love, hate (as in the feud between the Montagues and Capulets,) fate, and so on. Or, as mentioned above, they could be framed as conflicts, i.e. "love vs. hate," or "fate vs. free will." Either way, these are, it seems safe to say, fundamental to the human condition, transcending even cultural particularity. More to the point for a literature teacher, they transcend time, making them recognizable and relevant today.
The underlying meaning of a literary work. The theme may be stated or implied. Theme differes from the subject of a literary work in that it involves a statement or opinion about that subject. Not every literary work has a theme. Some literary works have more than one theme.