This tool shows an outline of the source code.  Each box represents a statement.  If you select a full statement in the code, a corresponding rectangle lights up in green.  If you select a partial statement, the corresponding rectangle shows up orange.  If you click a box in the view, the corresponding code is selected.

You can hold CTRL or Shift to select multiple blocks.  You press the up and down keys to navigate to sibling statements.  Press left to navigate to the parent.  Press right to navigate to the first child.