What is securing the homeland? What duties are encompassed by the impossibly large Department of Homeland Security?

Even DHS doesn't know:

"Thirty federal entities — among them agriculture, education, labor, treasury and social security — are receiving “homeland security” funding. The actual Department of Homeland Security, created in the aftermath of 9/11, receives 52 percent of the “homeland security” money pie, according to the Tuesdareport."