With this option you can select a coordinate or an entity and a pair of crossed red lines will signal it in the graphical window.
Signal coordinate expects x,y,z (z asumed 0 if it is omitted)
Signal entities expects the entity integer id of an existing entity
geometry view: point, line, surface or volume
mesh view: node or element
coordinates or entities could be also specified clicking in screen.
The Superpose Lines option (in the Contextual menu) is useful when in render mode. Depending whether it is set or not, the crossed red lines will either be in front of the object or partially hidden by the model.