You can rate an application to help organize the applicants you're interested in following up with. Ratings are private to the individual poster account and are not shared with the applicant or other businesses.
To rate an application, click on the hamburger navigation in the top left corner. Click the Your Jobs panel.
Your Jobs panel in side-navigation
Find the job post you are interested in and click All Applicants, or New. Look for the applicant name you are interested in, then click the blue arrow to the far right of the applicant's name.
Applicant Job Row in All Applicants list view
You are now in the Applicant Detail View. From here you can rate an application by clicking the star icon located in the Action Bar, to the right of the applicant's name.
Star icon in Action Bar