WHat she mean is that you cant restore/revert a database back to a past time without at least a full backup (.bak file) from that date or a backup from before that date and log backups taken after the backup and upto the required point in time?

Is that clearer? Think of it like this, if you have a calendar, you cant go back to see what you were doing in May if you have thrown away every page older then a week ago.