Demonstrative adjectives (this, that) are used to indicate a specific noun or nouns. In Italian, they must agree with the noun(s) in number and sometimes gender: questo, quella, quei ….
Demonstrative pronouns (this one, that one, the one[s], these, those) refer to a previously-mentioned noun in a sentence. Italian demonstrative pronouns are more complicated than their English counterparts, because there are two different sets and because they must agree in gender and number with the noun they replace.