A metaphor is a figurative way of expressing words by making a comparison between things that are not alike but have something in common. It is almost similar to a simile, except that metaphors make comparisons without the use of "as" or "like".
In the given expression <em>"your problems are a can of worms"</em>, the words mean that the problems get more complicated or are full of other messier problems. So, just like literally opening a can of worms that will wiggle their way out of the can and move in all directions, metaphorically speaking of "a can of worms" will mean problems that are all over the place.
