This was last year:
Tech companies set up booths and the students come by the booths learning about what it is like to work in a tech company. Colleges and Universities that offer undergraduate computer science majors also set up booths to recruit these students to attend their schools and major in computer science. And extra-curricular programs that offer computer science education after school, weekends, or over the summer also come and set up booths to solicit interest in their programs.
It is the one time each year when all the stakeholders in the NYC K12 computer science movement come together and it is a fantastic day. I have attended every year and I plan to attend again this year, in our fifth year.
If your company wants to help build a pipeline for a more inclusive and diverse talent pool, you should come and host a booth at the fair.