Yes, ad fraud. We're talking about endymi0n's claim in https://news.ycombinator.com/item?id=25429278 that without IDFA preventing ad fraud is (a) harder and (b) disproportionately harder for smaller players.

As for why you should care about it, see my response to thesuitonym below: https://news.ycombinator.com/item?id=25431866