who can tell me what's meaning of forward market efficiency and its calculat finance, general, meaning, forward, efficiency

ZEROKIKI

Who can explain this calculation to me (it relat to general efficiency of forward mkt):

E_tS_t+n =a+bF_t^t+n+E_t

I know the last E means the random of spot rate change, but dont know what the other Alphabet means, I have check lot of international finance book, but no explanation.