explanatory notes; introductory remarks; usage guide
Refers to the section at the beginning of a book, dictionary, or manual that explains conventions, symbols, abbreviations, and how to use the work.
If you read the dictionary's explanatory notes, you'll understand what the symbols mean.
Used for the explanatory key on maps, charts, diagrams, and drawings that defines symbols, colors, and patterns.
See also: キャプション
I looked at the map legend and saw that this line indicates a highway.