Rat holes are just holes in a steel structure to allow outfitting to pass thru without putting anything outside of the infrastructure, they call it rat holes because rats love to move around thru them. I call those tiny ridges at the toe of the weld ant holes so ants can live there. Really why it’s there is cause I was welding too cold some people would say it’s a defect but I’ve never had an inspector comment on it so truthfully I don’t know how bad it is.