It's all over the place, but generally it's just an effort to discredit what Columbus did by pointing out that Leif Ericson was there first. Of course, that doesn't make it less impressive, since it's not as if the Vikings told anybody. Columbus still was making a dangerous voyage that few others would try, and he's the one who actually "dis-covered" (as in, revealed to the Old World) the New World. Ericson may have been there first, but he left it "covered."

Alternatively, some truly wacky people try to claim that neither Ericson nor Columbus matter because the "Indigenous Peoples" discovered it first. Which once again, makes little difference, because those "discoverers" didn't maintain contact with the Old World.