The definition of a "good score" always depends on what college you are applying to, but I can give you some generalizations that will help you think it through. My class, High School Testing - Simply Explained, is sure to help you figure out all of the ins-and-outs to high school testing.
The ACT is scored on a 1 – 36 scale
"Average" score is about 20.
"Good" might be in the 20-26 range.
"Great" might be in the 24-27 range.
"Best" might be in the 28-36 range.
Or put another way,
"Average" will have limited chance for scholarships.
"Good" might have a good chance for scholarships.
"Great" might have a great chance for scholarships.
"Best" will have the very best chance of college admission and scholarships.
But each college is different, and each college has different average ACT scores they will accept. So look at the details for each college you are considering. Compare your ACT test scores to the average ACT scores of each college. That will tell you what a good score is in each particular college.
At any college, test scores are just ONE of the things they consider when they make their decision. They will be looking at your transcript, GPA, course descriptions, activities, reading list, essays, and letters of recommendation. All you have to do is your own personal best on the test - you don't have to get a perfect score just your best score for you.
Help your child get the best possible SAT or ACT score with the least possible stress. My article, How to Ace the SAT or ACT can explain how. You can also find out loads of information in my book, The HomeScholar Guide to College Admission and Scholarships. Get it HERE.