This is the 62nd most frequent Catalan word.
si
"Si" in Catalan means "yes" in English.
In this sentence, 'si' is used as a conditional conjunction meaning 'if'.
Si plou, agafa un paraigua.
If it rains, take an umbrella.
Here, 'si' introduces a subordinate interrogative clause, 'if' in English.
No sé si vindràs a la festa.
I don't know if you will come to the party.
In this context, 'sí' with an accent means 'yes', indicating affirmation.