The phrase "referral source" on a job application relates to how you happened upon the open position. Employers like to know what recruiting strategies and tactics produce the best job candidates; therefore, the information you provide in this section of the application is for employer recruiting purposes. It doesn't usually impact your employment, though a personal referral is often beneficial.
Referral Source Examples
In some cases, employers list several options for referral. Otherwise, you must indicate the best response. Typical referral sources include radio or television commercials, websites, help wanted ads, friend or family member, current employee and search engines. It is especially important to mention if you were referred by someone the hiring manager views favorably.