These terms refer to two basic reading skills that students should practice during an English language course. Skimming refers to the technique used to gain an overall idea of the content of a document or article by quickly looking through it. In contrast, scanning refers to the technique used to find specific pieces of information from within a document or article without having to read the entire text.
It is important for students to practice and master these reading techniques as they have many uses in the real world, as well as when taking exams and assessments. Also, simply learning that it is not always necessary to be able to read all the language you are presented with is somewhat of a relief for many students.