Those are placeholders. "We don't know what this sound is supposed to be, so we plop h+number there and call it a day." You'll see some reconstructions using *ə₁ *ə₂ *ə₃ instead, same deal.

That said, the Anatolian languages (Hittite, Luwian etc. - the whole branch is extinct) preserved a few of those laryngeals; compare for example Latin ⟨ouis⟩ and Hittite ⟨𒇻𒅖⟩ ḫāwis, from PIE *h₂ówis (sheep). Since Anatolian split way before the other languages, this makes me wonder if they weren't vocalised already in Late Proto-Indo-European.