Contrasts are where the author puts two opposing (opposite) ideas together to make the difference seem even bigger. There are a few different reasons they might do this. Look at the example below:
The author here is contrasting the idea of politicians spending a lot of money on expensive meals against the idea of homeless people not even having anywhere to sleep at night. Think about the effect of putting these two things together. Whilst these two things on their own seem like a normal (but terrible) part of everyday life, once you put the two together it seems totally unacceptable. A sense of shock is also created at just how different the lives of the homeless and politicians are.