Boosts are a way for you to get more applicants faster by advertising your listing to hyper targeted audiences on Facebook and Instagram. To add a boost to your listing go to your listings page and click on the manage button for the listing you want to boost.
In your dashboard, you will see a button to edit the listing itself. Click on this and you will see the page that you initially filled out in order to add the listing. There is a section on the left called Extras - that's where you add, edit or pause boosts.
Read more about boosts here.