The phonemes that particular letters or combinations of letters typically represent. Most languages need to be adjusted every 50 years or so to maintain the stability of grapheme-phoneme correspondences. English has not been reformed or adjusted for 900 years, which accounts for the large number of inconsistencies and irregularities found in its orthography.
See Decoding ability, Grapheme, Irregular words, Orthographic reading skills, Orthography, Phoneme