You may want to move this threshold. False positives (legitimate emails erroneously predicted as spam) are likely to cause more harm than false negatives (spam emails that are not identified as spam), as we might miss an important email, while it is easy to delete a spam message. In this case, we could require a higher threshold (probability) that a message is spam before we move it into a spam folder.
ROC means Receiver Operating Characteristic. It's an historical term from WW2 that was used to measure the accuracy of radar operators.
This is a single curve that captures the behaviour of the classification rate when varying the classification threshold.
The goal is to have:
False positive / True positive
Sensitivity / Specificity
Area under the curve (AUC)
A ROC curve for a J48 algorithm.