This video explains how refugees are vetted before entering the United States. From the description: ATTN: spoke with Homeland Security Secretary Jeh Johnson and his office, who explained the rigorous screening process refugees of the Syrian Civil War must go through to enter the United States of America. Watch here to better understand the process.
This UNHCR fact sheet shares some important information about how refugees are selected for resettlement. You can view the PDF version here: US Resettlement Fact Sheet – UNHCR [Source]