What means "Returns the specified element of the top-level mapping"?

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In the camera calibration tutorial (link: http:// docs.opencv.org/2.4/doc/tutorials/calib3d/ camera_calibration/camera_calibration.html# ), in part 1. Read the Settings) there is the line:

fs["Settings"] >> s;

which I don't understand. In the reference to the class FileStorage, of which fs is a member, [ ] is overloaded by:

FileNode    operator[] (const String &nodename) const

which is described by

Returns the specified element of the top-level mapping.

1. What does this description mean?
2. Why in the definition of the overloading, there is a "const" after "...&nodename)" ?
3. What is the operator >> doing? By default it's the bitwhise shifting, right? What would bitwise shifting do here?

Thank you in advance for your answers.