Remember, he said, "If it's a legitimate rape, the female body has ways to try to shut that whole thing [pregnancy] down." He was re-asserting the distinction, popular among abortion-banners, between "real" rape (what he meant by "legitimate" rape) and some unspecified but putatively-large category of rape that isn't really rape, just women claiming to be raped when it was actually consensual or they're just mad at a guy or they just need an excuse to get an abortion or whatever it is that these guys (it always seems to be guys) have in mind when they make this distinction. Abortion-banners know that rape is a case that undermines their stance in the eyes of many who might otherwise favor it, so they have to minimize rape as an issue, and one way of doing that is to claim that real rape is much rarer than people think.
What makes Akin's formulation distinctively disturbing is the claim that his (wholly imaginary) natural anti-conception mechanism operates exclusively in cases of "real" ("legitimate") rape. That is, if you want an abortion after a rape, the very fact that you got pregnant shows that it wasn't a real rape. If it had been a real rape, you wouldn't have gotten pregnant. So there. No abortion for you, and stop making a big deal out of nothing.
That's what he was really implying.
This man is a major-party candidate for a Senate seat.