Demonstrative pronouns are used to emphasise which specific thing (of multiple possibilities) you are talking about – in English we would say 'this one, or that one, or these ones, or those ones.
They generally follow mention of the possibilities you are emphasising and only exist in sentences of one of three situations:
Notice that the first example uses ceux and the others use celui. The demonstrative pronouns match the object they are talking about, and there are 4 options:
Reminder: pronouns, including the demonstrative pronouns above, are when you do not mention the noun e.g. I like this one. If you do mention the noun it becomes instead a demonstrative adjective i.e. ce, cet, cette or ces e.g. I like this house .