JunkMatcher Howto: Log
Firstly, note that this is only one of the two ways to correct JunkMatcher: you can also make/revert corrections directly from within Mail.app, with the added benefit that your correction will simultaneously train Mail.app's built-in filter.
Inside the Log window, every entry gets a "Junk?" checkbox - if it is checked, it means that the message has been classified by JunkMatcher as junk (the entries are also color-coded so you can spot the clean/corrected emails more easily).
(Also note some of the messages are marked by a shade of light blue in the Received Date column - it means that they have been used to train SpamBayes)
You can correct JunkMatcher's mistake by changing the verdict of an email: just click on the "Junk?" checkbox to reverse the verdict. The first click will mark the entry "corrected" (in different color), and a second click will revert the correction (no color).
When you make a correction by changing the verdict of an email, JunkMatcher will update the statistics of all of the tests that have been conducted on that email. In addition to that, correcting a false positive (i.e., a clean email was falsely classified as junk) will:
Conversely, correcting a false negative (i.e., missed junk) will add all of the URLs mentioned in the email into the Bad Site collection, and train SpamBayes with the email as spam.
If you decide to revert your correction, all of the above will be reverted as well (e.g., SpamBayes will be untrained etc).
Look closer at the screenshot here: there is a text field below the log table (to the left) that allows you to enter the number of days the log will be kept; e.g., if you set it to 5 days, on the 6th day your log will be automatically emptied.