The enterFunction bindkey is active during an "enter function", that is, a function that accepts input such as when deleting objects, the command is accepting co-ordinates for the location of the object to delete, or when copying, the location of the object to copy and location of the new copy, and of course design entry commands such as when creating polygons, paths, rectangles etc. This allows the user to define a different action when in one of the 'entry mode' commands.
You can define a bindkey to be active during an enterFunction by appending "EF" to the key definition, here's an example from the documentation:
hiSetBindKey("Schematics" "None <Btn1Down> EF" "mouseAddPt( )")
Hopefully this answers your questions.