The key that says what a map's marks mean. Also a story handed down — both make something readable that otherwise is not.
Medieval Latin legenda (things to be read), from legere (to read, gather); first a saint's life read aloud on the feast day, then any handed-down story, then the key that makes a map's marks readable.
What makes a map's marks readable, and what makes an old story worth handing down — both take something opaque and make it legible.