Themes are essentially the reoccuring ideas that pieces of literature expand and develop.
Common themes include:
The struggle of the individual against society or nature
time
death
dreams...
it's kind of like the moral or technically the things that make up the book e.g the themes in twilight novel are love, revenge, death e.t.c do you get?
When discussing literature, theme means the central idea around which the poem, story, play, novel, essay or a book is developed. A theme is as essential as water is to sea.