síadverb

Sí means yes in English. It is an adverb.

yes

Sí, él está en casa.

Yes, he is at home.

Sí, yo voy a la casa.

Yes, I am going to the house.

"¿Te gusta viajar?" "Sí."

"Do you like to travel?" "Yes."

"¿Ella juega tenis?" "Sí."

"Does she play tennis?" "Yes, she does."