Demonstrative pronouns are used to replace nouns.
Example (first sentence is using a demonstrative adjective, second is using demonstrative pronoun):
Questo giornale è di ieri. (This newspaper is from yesterday.)
Quello è di oggi. (This one is from today.)
Demonstrative pronouns match the gender and number of the noun they replace.
These are very similar to demonstrative adjectives with 2 differences:
Demonstrative pronouns do not have the contracted forms with an apostrophe like quest' and quell'
When talking about something far away, there is only one demonstrative pronoun for M singular (quello, no quel), and one for M plural (quelli, no quei or quegli)