This is a micro econometrics question, but I do not understand some of the symbols used in the answer (the red parts)

Could you please tell me what does the part that is highlighted in yellow represent?

12. The question is how education, age and some family circumstances are related to female labor supply. The dataset is an individual cross-sectional sample of women between age 25 and 60,57 percent of whom work. Labor supply is modeled as the probability of being employed. a. Write down a latent variable model for a probit model on the probability of employment, with education (in years), age, and age squared on the right-hand side. Assume exogeneity of all right-hand side variables. Y=b0+b1 educ +b2 age +b3 agesq +u;uxN, E[y>0 educ, age ]=(b0+b1 educ +b2 age +b3 agesq ) 12. The question is how education, age and some family circumstances are related to female labor supply. The dataset is an individual cross-sectional sample of women between age 25 and 60,57 percent of whom work. Labor supply is modeled as the probability of being employed. a. Write down a latent variable model for a probit model on the probability of employment, with education (in years), age, and age squared on the right-hand side. Assume exogeneity of all right-hand side variables. Y=b0+b1 educ +b2 age +b3 agesq +u;uxN, E[y>0 educ, age ]=(b0+b1 educ +b2 age +b3 agesq )