why are fe_frombytes function of openssl in curve25519.c like this?

Question

static void fe_frombytes(fe h, const uint8_t *s)
{
    /* Ignores top bit of h. */
    int64_t h0 =  load_4(s);
    int64_t h1 =  load_3(s +  4) << 6;  
    in