This is the 317th most frequent Finnish word.
joko
"Joko" means "either" or "already," depending on context.
Here, 'joko' is used to inquire whether the action described (eating breakfast) has been completed.
Joko olet syönyt aamupalan?
Have you had your breakfast yet?
In this sentence, 'joko' is first of a pair in 'joko...tai' and serves to list two alternatives.
Voimme mennä joko elokuviin tai teatteriin.
We can either go to the movies or to the theater.
Here, 'joko' introduces an option between two time frames ('now or later').