A snowball idea is based on the premise of a snowball, rolling down the hill. It starts off as a tiny little snowball, but as it keeps rolling and rolling, it keeps adding more and more snow, until it is one gigantic snowball, and when it hits, it is a big mess.
It applies to comics (and movies and TV and everyday life) in much the same way.
Someone will suggest something, often not even being serious. The idea then is sent down the hill, and if no one cuts it off, it just keeps adding snow and gaining momentum until it is so big that NO one can stop it.
Grant Morrison's "Batman protocols" (and the Marvel equivalent, the "Xavier protocols") is a perfect example.
Once Morrison brought the idea up in a Wizard JLA special, you KNEW the idea would be used by SOMEone later on, you just did not know where and by who.
It was a snowball idea.
A current snowball idea is the whole "return of Jason Todd" idea. Once Loeb brought it up in Hush, even if he meant it as something to not be taken seriously, it began its snowball trip. When it will hit and make a big mess is the only question now...
EDITED TO ADD: Note that Jason Todd DID end up returning later that year! I think the current "snowball idea" is Spider-Man and Mary Jane's marriage ending.