This is the 11th most frequent Finnish word.
joka
"Joka" means "who," "which," or "that" (relative pronoun).
In this sentence, 'joka' introduces a relative clause, describing 'Tyttö'.
Tyttö, joka istuu puun alla, on minun serkkuni.
The girl who is sitting under the tree is my cousin.
'Joka' in this sentence serves as the subject of the relative clause referring to 'Jokaisen'.
Jokaisen, joka haluaa osallistua, tulee ilmoittautua etukäteen.
Everyone who wants to participate must register in advance.
Here, 'Joka' functions as a quantifier, emphasizing the recurrence when talking about 'kerta'.