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?**

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https://preview.redd.it/txyg36c4m94c1.jpg?width=1032&format=pjpg&auto=webp&s=dc929c482885d4a28cf90c8ca0617b3129aa1ae2