It most often describes the issue of low-income/minority education in the United States; that is, that Blacks and Latinos and students from poor families perform worse in school than their well-off White and Asian peers. SAT scores broken down by family income show when students have similar family incomes, Black and Latino students still score lower than Whites, and Whites score lower than Asians with similar incomes.
The achievement gap can be observed on a variety of measures, including standardized test scores, grade point average, dropout rates, and college-enrollment and -completion rates. Explanations for the phenomenon -- and levels of concern over its existence -- vary widely, and are the source of much controversy, especially since the effort to "close the achievement gap" has become some of the more politically prominent education reform issues.