Demonstrative pronouns replace a noun already mentioned, agreeing with its gender and number: celui (masculine singular), celle (feminine singular), ceux (masculine plural), celles (feminine plural) — all meaning roughly "the one(s)". They're rarely used alone and almost always followed by -ci/-là, de, or a relative clause.
With -ci and -là they distinguish "this one" from "that one": je préfère celui-ci (I prefer this one). With de, they show possession without repeating the noun: ma voiture et celle de mon frère (my car and my brother's — literally "that of my brother"). With qui/que, they introduce a description: celui qui parle est mon professeur (the one who is speaking is my teacher).