A "sacrificial lamb" in this context is someone who agrees to be on a ballot just to put someone on the ballot. The more common term is paper candidates. Typically most don't even campaign or anything, and certainly no money is ever spent on them. It's the sort of thing where you just kind of sign a form and then forget about it.

It's important for parties to establish presence everywhere you can if you want to be taken seriously, especially in a two party state, but a lot of the time "everywhere" is "The parts of Louisiana so deep red that they hunt Democrats for sport." No one is actually planning for even the chance of a win in these places.

However, the midterms were such an ungodly bloodbath for the GOP that the Dems are winning in places they weren't even trying to. A lot of paper candidates just woke up and learned they have new jobs.