It implies that something exists but in such a small amount that it's essentially insignificant.
Here are some examples of how it can be used:
* "The chances of winning the lottery are virtually nil." This means winning is incredibly unlikely.
* "The impact of the policy change on the economy was virtually nil." The change had almost no effect.
* "There was virtually nil evidence to support the defendant's claims." There was barely any proof.
Essentially, "virtually nil" emphasizes the extreme smallness or insignificance of something.