Any time you didn't appear for militia muster you were fined. It doesn't mean that the ancestor wasn't otherwise in good standing with the military, sometimes you missed a muster because of a sick family member or crops that needed tending. Often, these "fines" were issued and waived when the militia member came to the next drill and had a good reason. But monthly drills weren't always recorded because they were "ordinary," and often, we only have remnants from the past for events that were not ordinary.
If you could post a link and/or the full text and title of your record it would help us to understand it better.