Each threshold has it own unique FPR and TPR, so each point on the plot is a specific threshold FPR and TPR for each of the class ROC curve. The threshold determines the number of TP,TN, FN, FP, which eventually results to a unique TPR and FPR for each threshold, and then they are plotted for each class. So, practically, we select a set of thresholds to evaluate the model on.