Linguistics/phonetics term. Note the reading:
塞 has three readings (sè 'blocked / to stop up', sāi 'to stuff', sài 'frontier pass') — here it's sè ('to block/stop'), giving the technical meaning 'stop consonant'. Also called
爆发音 (bàofāyīn, 'explosive sound') or
破裂音 (pòlièyīn, 'rupture sound').