The passage is "The Pope acts most rightly in granting remission to souls, not by the power of the keys (which is of no avail in this case) but by the way of suffrage"
He's talking about the souls in purgatory here, over which the pope has no power in an institutional sense (namely through the sale of indulgences), but only through"prayers for the dead" - per modum suffragii. So the keys represent the pope's might, which, as Luther claims, is limited to earth (super terram) and does not - as the church says - extend to heaven, hell or the purgatory.