Because the buffer regions consist of the acid and its conjugate base, it can resist pH changes when base is added until the next equivalent points.
bɪkˈʌz ðə bˈʌfɚ ɹˈiːdʒənz kənsˈɪst ʌvðɪ ˈæsɪd ænd ɪts kˈɑːndʒuːɡˌeɪt bˈeɪs, ɪt kæn ɹᵻzˈɪst pˈiː ˈeɪtʃ tʃˈeɪndʒᵻz wɛn bˈeɪs ɪz ˈædᵻd ʌntˈɪl ðə nˈɛkst ɪkwˈɪvələnt pˈɔɪnts.