In the original game of Icehouse, a floss call is a ruling, usually by a referee, to determine exactly where an attacker is pointing (i.e. exactly what its attack hits). This is usually done by holding a piece of thin string taut over the attacking piece to better visualize the attacker's line of sight, and what piece that line extends to touch. A laser line generator works as well, if not better, but they're kind of expensive.
Metaphorically, a floss call is kind of like a photo finish -- it's an outcome that cannot easily be determined by the naked eye, but requires some more exacting measuring device.
Any game in which flat pieces point at things precisely may require a floss call. Games which are known to sometimes require floss calls include: