In a literal sense a can of worms is a can filled with worms that are either real or more commonly compressed worm-shaped pieces of paper that are made so as to jump out when the can is opened (as a prank).
The phrase is used as "The Can of Worms" as a local name for the
intersection of Interstates 590 and 490 on the east side of Rochester, New York.
As a saying it is generally used to explain a
significant complication (for better or worse) of a situation or topic by referring to it, this situation or topic may be a subset of a topic or subject that preceded it. This saying is sometimes used following phrases such as: "open up another", "a whole other", "nothing but a" or similiar
Saying Example 1. Well car mechanics is one thing, but truck mechanics is a whole other can of worms
Saying Example 2.
I don't want to start another course at
university because thats going to open up another can of worms
Saying Example 3. The shortcut seemed easy enough
on the map, but when we went through it was nothing but a can of worms