The title is supposed to describe the main idea of an essay in the most clear and instigating way possible.
If the title of your essay is "Politicians are not supposed to be trusted", it is seriously expected of the essay to explain that statement.
An essay that has "cheese and wine" as title will hardly talk about chocolates, although there are some clever ways of using the title to bring out something different and unexpected as long as you can make it all make sense in the end.
For example, "The government robs you" is intriguing enough, but it can be a clever way of talking about how we all must pay taxes whether or not we agree with the governmental policies.
Even when that is the case, though, the title will dictate what your essay is about.