Safety bearings are a common feature in short distance orienteering and adventure races. The idea of a safety bearing is that it can be used when you are lost. It will enable you to find your way back to civilization or some feature that will make it easy to find yourself. For example, it may bring you to major roads, nearby housing developments, etc.
The safety bearing will usually be given as a simple cardinal direction (north, south, east, west) so that it can be followed by someone with little or no experience or someone who is disoriented.
In the example above, the safety bearing would be WEST. If you travel west from any point on the course you will end up on St. Mary's Street. It is assumed that, once you've made your way to this major road, you can be more easily found or can find your way back to the start/finish line.
One important consideration for a safety bearing is that it will probably not be a comfortable trip following the bearing. You will probably be bushwhacking. However, if you persist, it will get you back to a safe point.
If you meet an obstacle that shouldn't be traversed, or particularly difficult terrain, you can (and should) go around the feature and then continue on the safety bearing. You wouldn't want to go through a major body of water obviously!
I will post another article on how to follow a bearing.
If you are AT ALL UNSURE of your ability to successfully follow the safety bearing, the best policy, when lost, is to stay where you are until you are found.