1. The actual tail of a calf: This is the obvious meaning. The tail of a young cow, typically called a "calf" until they're about a year old.
2. A slang term for something that is small or insignificant: This meaning is less common and likely to be understood only by those familiar with the slang. It's similar to phrases like "a drop in the bucket" or "a blip on the radar."
If you can provide more context, like the situation where you heard the term, I can help you determine which meaning is intended.