An earlier example (הספר יפה) was translated as "the book is beautiful." I had put down "the beautiful book" and got it wrong. That sentence has the definite article ה , whereas this example is indefinite. Still, the lessons are a bit inconsistent -- without an explicit verb for to be, it's not always clear to those of us who are learning whether you mean "good girl", "a good girl", "a girl is good", or "a good girl is".
The difference is when equating one thing to another e.g. a girl is good, a dog is an animal (in this case "girl"/"dog" is the subject) ילדה היא טובה כלב הוא חיה
Unlike when you're describing the noun like a good girl, a brown dog. ילדה טובה כלב חום
If you think of these as full sentences, rather than fragments you'd get "she's a good girl" (in this case "she" is the subject and you are describing her), which would translate as היא ילדה טובה