Rotating a log (using some logrotate utility) means that after a certain configured time, the logs specified (usually syslog, messages, etc) will be compressed, suffixed with a dot and a number (i.e. /var/log/syslog.1). This may differ depending on software for rotating the logs. This way you can keep old logs for quite long, but since they get compressed you still save some diskspace. It is used a lot by different distros today.