Hola Wardo1234: Some Spanish schools teach that you do need the redundant direct object pronoun; others do not. Duolingo sometimes uses it, other times does not. Personally, I learned NOT to use it in language schools in Mexico.
So if the sentence were, "She understands the document", it would be, "Ella entiende el documento", but because "el maestro" is a person and not an object, it requires a "personal a", making it "al maestro" (or "a la maestra"). Correct?