Off course navigation is used to account for unknown variables in course to allow you to find your destination. Principally used prior to the introduction of electronic navigation it allows one to find one's destination despite the unknown factors - generally - the accuracy of the course steered or the drift to one side introduced by current or wind. Simply put - instead of aiming directly for your destination you deliberately aim off to one side of it so that once you have travelled the correct distance towards the destination - your target must lie to a known side.