Questi is a third-person demonstrative pronoun used to refer to persons close to the speaker, or just mentioned.
It functions most of the time as the subject of a verb in the sentence and is used in the singular masculine form only. For example:
Ho incontrato Marco e Filippo, questi (that is, Filippo) mi ha invitato al suo matrimonio / I’ve met Marco and Filippo, who invited me over to his wedding
DO NOT MISTAKE the demonstrative pronoun questi (third-person masculine singular) with the demonstrative adjective / pronoun questi (masculine plural form of questo).
Examples:
masc. sing. pronoun —> Questi è il famosissimo scrittore Dante Alighieri / This person is
the very famous writer Dante Alighieri
masc. plur. adjective (questo) —> Mi ricorderò sempre questi strani palazzi di fronte alla mia
finestra / I will always remember these strange buildings in front of my window
