This is the 4368th most frequent Spanish word.
sé
"Sé" means "I know" (from the verb "saber") or "Be" (informal command from the verb "ser").
In this sentence, 'sé' is the first person singular conjugation of the verb 'saber', meaning 'to know', indicating knowledge of something.
Yo sé que hoy será un gran día.
I know that today will be a great day.
Here, 'sé' is the imperative form of the verb 'ser', meaning 'to be', used to give an affirmative command.
Sé amable con los demás.
Be kind to others.
Again, 'sé' is used as the verb 'saber' in first person singular, indicating lack of knowledge of the answer.