As part of the article's literature review, it mentions two kinds of achievement gaps:

• students from high-income families show better educational outcomes compared to students from low-income families (i.e. over a summer between school years, high-SES students actually show improvement in reading skills while low-SES students decline in reading skills)
• school closures over an extensive period of time can cause a decline in educational outcomes like test scores or success in classes (they cite the 1916 polio pandemic as having caused students to struggle in academics due to missed time in class)