This is a very popular concept that people talk about.
You hear it alot in the news.
The achievement gap relates to the differences that people experience in education. This can relate to the differences in education because of people's races, because of people's genders, and so on.
In addition, you hear alot about it to with people and their households and towns. For example, you hear about the differences in education in richer and wealthier towns versus more low income towns and this relates to how kids achieve and how well they do in school, on their tests, and so on.
So many different factors can affect the achievement gap.