This is the 1084th most frequent Spanish word.
s
In Spanish, "s" is the 19th letter of the alphabet and represents the "s" sound.
Here, 'es' is a conjugation of the verb 'ser', meaning 'is', to describe a state of being.
Ella es mi amiga principal.
She is my best friend.
In this sentence, 'sé' is a conjugation of 'saber', meaning 'I know', and 'está' is 'is' indicating location.
No sé dónde está mi libro.
I don't know where my book is.
Here, 'son' is another conjugation of 'ser', used for describing continual or general states of being.