Reading a ROC curve
A receiver operating characteristic (ROC) curve plots the true positive rate (sensitivity) on the vertical axis against the false positive rate (one minus specificity) on the horizontal axis, tracing how the two trade off as the classification threshold sweeps across its full range. Each point on the curve corresponds to one threshold. A curve that hugs the top-left corner indicates a test that separates classes well, while a curve along the diagonal indicates performance no better than chance.
What AUC summarizes
The area under the ROC curve (AUC) condenses the whole curve into a single number between 0.5 and 1.0. It has an intuitive reading: the probability that the test ranks a randomly chosen positive case higher than a randomly chosen negative one. An AUC of 0.5 is chance and 1.0 is perfect discrimination. AUC measures discrimination, the ability to rank cases, and is independent of any single chosen threshold; it does not by itself describe calibration or the clinical consequences of false results.
This is a statistical explanation for educational use. Choosing an operating threshold for a real diagnostic test involves clinical judgment and is not implied by AUC alone.