According to the screening theory, employers always want to choose people with appropriate abilities from a large number of job seekers to fill vacant positions. However, he met job seekers in the labor market and didn't know their abilities. Although he can't directly understand the productivity of job seekers, he can learn some tangible personal attributes and characteristics of job seekers. One is inherently immutable, such as gender, race, family background, etc., and the other is acquired, such as education level, marital status, personal experience, etc. The former is called "sign" and the latter is called "signal". Employers can understand the ability of job seekers through signs and signals, especially educational signals. This theory regards education as information and pays attention to how employers choose workers in the labor market. Its viewpoint is: it agrees with the theory of human capital that the education level is proportional to the wage income level. The higher the education, the higher the salary level.