The symbol H stands for theoretical laryngeal sounds, three of them denoted /H1, H2, H3/ or /h1, h2, h3/ or, sometimes, in other ways. These sounds were lost, in Proto-Indo-European, following the emergence of Proto-Anatolian; therefore, no Indo-European language family except Anatolian reflects laryngeals in any direct fashion. In Hittite, at least the second laryngeal is preserved in the sign for h ([kh] in the cuneiform script borrowed from Old Babylonian Akkadian): i.e., Hittite h usually corresponds to the lost /H2/. Where prehistoric laryngeals were lost, vowels in the non-Anatolian languages often correspond irregularly; such irregular correspondences led Ferdinand de Saussure (in 1878, long before the discovery of Hittite) to hypothesize lost co-efficients sonantiques. They are called laryngeals because they are thought to have been articulated in the region of the larynx, in the throat, like the English glottal fricative [h].
According to recent hypotheses, H1 "colored" a contiguous vowel to /e/; H2, to /a/; and H3, to /o/. Before coloring, the sound sequences would have been /H1e, H2e, H3e/ or /eH1, eH2, eH3/; and after coloring, /H1e, H2a, H3o/ or /eH1, aH2, oH3/. As independent sounds, laryngeals are preserved only in Hittite; in other languages, which evolved from PIE following the Anatolian split, their disappearance left reflexes such as vowel lengthening when they followed the vowel.
All three laryngeals are exemplified as PIE phonemes on a laryngeal roots page to demonstrate their theoretical position in the sound inventory of Proto-Indo-European; roots on that page are spelled with initial or final /H/ but link to pages that spell those roots with the vowels /e/, /a/, or /o/ -- vowels once reconstructed as basic to PIE but now thought to be the results of laryngeal coloring.