We already know this letter: which is pronounced pretty much the way we pronounce the letter p. There’s some air behind it, but not much. It’s romanized as p or b, depending upon its sound..
This letter, has an extra-strength puff of air behind it. It’s always romanized as p.
In older versions of Korean romanization, this letter was romanized as p’; now it’s just p. This is why you really have to learn the alphabet, and cannot rely on Romanization. If you see the letter p in a Romanized word, you can’t tell if you have a or a .
And this letter, has no air; it's the p of spin. Your throat sort of tightens up when you say it; the technical term for this is that the letter is glottalized. It is romanized as pp.