Light Curve Graphs (I Need Urgent Help)

So I’ve been looking at light curve graphs trying to understand them for this thing I do called science Olympiad. Anyway, I understand them kind of so far. The x axis is the time it takes for the planet to pass in front of the star (I think) and the Y axis is the lumonsity as seen by the observer. However, I noticed there were negatives on the graph which make no sense.

**Are my units right and WHY ARE THERE NEGATIVES?**