«ce sont» is the plural form of «c'est». «ce» used in this manner is an indefinite, invariable pronoun that is used mostly, but not exclusively, with «être» (instead of cela/ça). 

Of course «ce» can also be an adjective - «ces» is then the plural form,  and is followed by a noun, not a verb.

C'est, ce sont = this is, these are (demonstrative pronouns)

demonstrative-adjectives-ce-cet-cette-and-ces-mean-this-that-and-these-those

That are relaxing holidays. -- Ce sont des vacances relaxantes.
These holidays are relaxing. -- Ces vacances sont relaxantes.

Notice how, in the first sentence, ce does not refer to vacances but is a standalone subject of the sentence. In the second example, ces is a demonstrative pronoun belonging to vacances and needs to be matched to it.

That explanation was so enlightening. It is wonderful  when things make sense.