C'est, ce sont = this is, these are (demonstrative pronouns) : French language revision

Remember that in sentences it is/they are ... + article/determinant (un, une, le, la, les, des...) + noun/pronoun, you will use c'est or ce sont instead of il/elle est or ils/elles sont.
See C'est vs il/elle est: Saying it is

Now look at these examples:

Est-ce ta trousse? - Oui, c'est la mienne.
Is this your pencil case? - Yes, this is mine.

À qui sont ces livres? Ce sont mes livres.
Whose books are these? These are my books

C'est ma maison.
This is my house.

Ce sont mes maisons.
These are my houses.

Note that ce is used with the verb être as follows: in its singular form, c'est, means it is /this is /that is, and in its plural form, ce sont, means they are /these are /those are.

Both forms are used to refer to a thing or a person (i.e. a girl, a table, a house, ...), and they agree in number (singular or plural) with the thing they refer to.

BUT

When c'est is followed by an adjective or an adverb on its own, you NEVER use ce sont, even if the thing referred to is plural:

C'est pratique, les ciseaux.
They're practical, scissors.

Et leurs anniversaires ? - C'est bientôt !
What about their birthdays? - They're soon!

Note that if c'est is followed by an adverb AND a noun, then the noun is the one that matters!

Ce sont bientôt les vacances!
It's almost the holidays!

ATTENTION:

It is worth noticing that the first rule is not always followed rigorously by French speakers. You could indeed hear them using c'est when they should use ce sont:

Ce sont bientôt les vacances!
It's almost the holidays!

C'est bientôt les vacances!
It's the holidays soon!

However, they will never use ce sont for c'est !