Theme is the lesson an author wants us to learn from reading his/her literary work.
Themes are never directly stated in the literary work. They are to be inferred.
Theme might be a lesson that a character learns as a result of his/her decisions. Theme might also be a lesson the character learns from what is happening around him/her.
Common themes from literature you may have read include:
You may need to sacrifice in order to fight corruption. (The Hunger Games)
Working together can help you accomplish your goals. (The Maze Runner)
Our differences can make us capable of amazing things. (Divergent)
Themes should always be stated in complete sentences. If you use just a word or a few words, it can be unclear.
For example, if you say the theme is "love", then it could be a number of things: love is an amazing force, love can be terrible, love can destroy you... and so on.