As a reader, you are looking for themes in the text you are reading: ideas you can connect to the text's contents. But there is a difference between identifying a theme and reading the argument(s) presented by the text.
The argument is, basically, the reasoning behind the theme, in other words, the reasons given (developed, alluded to) for a particular take or stance (theme) on a topic (what the text is about).
It's immediately clear that texts will have different levels of complexity related to the complexity of the argument - or to the number of arguments present in the text.
For example, a simple poem for children about bullying (the topic) will tend to have a simple theme (bullying is bad) and an equally simple (if perhaps true, that is not the question here) argument (bullies are afraid and take their fear out on others).
A more complex text about bullying (topic) may have more complex themes (e.g. bullying is bad; bullies are bad people; bullies can't help themselves and should be supported; bullies suffer from their own behaviour), and more complex arguments (e.g. bullies may be afraid themselves, but they may also be bad people to start with; they may have problems at home; they may have the wrong ideas about collective living; they may have psychological problems)
Similarly, someone who says a particular text is 'about human nature', or 'about greed and ambition' only states the theme but says nothing about the argument, i.e. the particular stance, in the text, on that idea. For example, a text 'about human nature' could argue that 'human nature is by definition flawed' while another could argue that 'human nature is by definition good', as a third might argue that 'human nature is always stronger than culture'. It's obvious that there could be a great many different arguments for any theme.
Differentiating between topic and theme, and theme and argument, therefore helps you clarify which is which, and gives you more ideas about how to interpret a text. It then gives you much more material to discuss in class, since you are not limited to a simple argument (=answer).
Rather than ask 'What does the author mean?',, or 'What is the text about', ask: 'What are the arguments presented in this text for or against this particular topic and theme?'