In chapter 8 of Holes, the narrator gives us a detailed description of these dangerous creatures that like to hide in holes before leaping out at unsuspecting prey. There's a reason for this. The yellow-spotted lizard shares many of the characteristics of the boys at Camp Green Lake, who've been treated as little better than animals by the authorities. Like the lizard, the boys spend most of their time in holes; they can also be quite dangerous if provoked.
And it's notable that the lizards eat sunflower seeds, because that's what Mr. Sir constantly spits into the holes that the boys dig. Without knowing exactly what's going to happen later on in the story, we can already sense that this nugget of information will come to be significant in due course.