It’s the belief that when Jesus was crucified, he was being punished instead of us, that he died in our place — a bit like an older brother might protect his younger brother from the wrath of their father by saying he broke the garage window when it was actually the youngster who did it … only in this case, because it’s God who’s the father-figure, he knows full well who’s guilty and who’s innocent, but goes along with the scheme anyway, and the whole business gets a bit more brutal than having to pay for a broken window. The upshot is Jesus dies and you & I get off scot free, theoretically forgiven but in practice there’s no forgiveness necessary because someone else, namely Jesus, has taken the punishment — a very convoluted way of thinking, to say the least, but most evangelicals seem to manage it somehow.

If you’d like a longer explanation, check out J I Packer’s 1973 essay, The Logic of Penal Substitution:

The notion which the phrase ‘penal substitution’ expresses is that Jesus Christ our Lord, moved by a love that was determined to do everything necessary to save us, endured and exhausted the destructive divine judgment for which we were otherwise inescapably destined, and so won us forgiveness, adoption and glory. To affirm penal substitution is to say that believers are in debt to Christ specifically for this, and that this is the mainspring of all their joy, peace and praise both now and for eternity.

He goes on to make the analogy that we can eat a meal and benefit from it without needing to know precisely how the proteins & vitamins achieve that benefit for us.

It’s a nice thought to set against the latest 500 page book on the subject, anyway.