It helps us to compute log Pr(f * |e s,t − , a) by making explicit how each word contributes to words it aligns with.
to compute the probability that f is a translation of e, Pr(f |e), IBM Model 1 defined a generative alignment model where every word f i in f is aligned with exactly one word e a i in e, so that f i and e a i are word level translations of each other.