C'est, ce sont = this is, these are (French Demonstrative Pronouns)

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" to say it is/she is/he is in French

Now look at these examples:

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:

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

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:

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