This is the 543rd most frequent Catalan word.
sé
First-person singular present indicative form of "saber" (to know).
In this sentence, 'sé' translates to 'I know' in English and is the first person singular present indicative form of the verb 'saber', meaning 'to know'.
Sé que arribaràs tard perquè plou.
I know you will arrive late because it's raining.
Here, 'sé' is used in a negative form 'no sé', which means 'I don't know', again indicating the speaker's lack of knowledge in the present tense.
No sé com funciona aquest aparell.
I don't know how this device works.
In this example, 'sé' indicates the speaker's ability or skill, as a part of the more general meaning of 'saber' which encompasses 'to know how (to do something)'.