'son' means 'his' or 'her' or 'its' describing a masculine thing, as in 'son chocolat' - his/her/its chocolate.
'sa' means the same, but describes a feminine thing: 'sa fille' - his/her/its daughter
'ses' means the same, but describes a plural thing: 'ses yeux' - his/her/its eyes
These are all adjectives, because they are used to describe a noun (chocolat, fille, yeux). Specifically, they are called possessive adjectives because they indicate ownership.
In English as in French, the subject of a verb needs to be a noun or pronoun. So you can't use one of these demonstrative adjectives as a subject. 'Son aime nager' in French is as wrong as 'Its likes swimming' in English.
Grigri est le chat de Robyn. Grigri is Robyn's cat.
You can replace the subject noun, Grigri, with a subject pronoun (il/he), and you can replace the phrase showing ownership of the cat (de Robyn/Robyn's) with a possessive adjective (son/her)
Il est son chat. He is her cat.
But you can't say:
Son est il chat. Her is he cat.