Each entry lists a keyword identifying the visitor as a spammer. Once a visitor is identified as a spammer, all requests from this visitor will be treated as spam for the rest of the currently-reported month. Spam requests will be counted as usual in all reports, except the referrer report, to prevent spam referrers from appearing in the report as clickable links. Spamming hosts will also be highlighted in red color in the hosts report.
More information about defining spam referrers can be found here.
Please note: Only the Stone Steps version of the Webalizer is able to interpret this parameter.