The receiver operating characteristic (ROC) curve traces one classifier across every possible decision cutoff. Each point pairs the true positive rate with the false positive rate at that cutoff. Sliding from a strict cutoff to a loose one walks you along the curve.
Read it by corner. The bottom left is flagging nothing, the top right is flagging everything, and both are useless. The diagonal line is guessing at random. A curve that bulges up and to the left is separating the classes well.
The practical use is choosing where to operate. Find the largest false positive rate your team can absorb, read up to the curve, and take the cutoff there. One caution: with very rare positives the curve can look great while most of your flags are still wrong.
Rewriting in plainer words…
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