Brackets indicate concentration, such that [H2O] means the concentration of H2O. The acid dissociation constant Ka is generally used in the context of acid–base reactions.
bɹˈækɪts ˈɪndᵻkˌeɪt kˌɑːnsəntɹˈeɪʃən, sˈʌtʃ ðæt ˈeɪtʃ tˈuː ˈoʊ mˈiːnz ðə kˌɑːnsəntɹˈeɪʃən ʌv ˈeɪtʃ tˈuː ˈoʊ. ðɪ ˈæsɪd dɪsˌoʊsɪˈeɪʃən kˈɑːnstənt kˈɑː ɪz dʒˈɛnɚɹəli jˈuːzd ɪnðə kˈɑːntɛkst ʌv ˈæsɪd bˈeɪs ɹɪˈækʃənz.