A state guarantee is a collateral instrument granted for a loan by the state. In student loans it means that Kela will repay the loan to the bank if you are unable to repay it yourself. Remember, though, that Kela will collect the debt from you later. Since the student loan is state-guaranteed, you won’t need any other collateral for the loan.
You will need to apply for the state guarantee from Kela. After Kela has granted you the state guarantee, you can apply for the student loan from the bank of your choice.