We already know this letter: ㅂ 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, ㅍ has an extra-strength puff of air behind it. It’s always romanized as p.
And this letter, ㅃ 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 ㅃ is glottalized. It is romanized as pp.