A standing bonus is the extra cash given for the loyalty involved in the faction you're affiliated in (i.e Terran gives 75% standing bonus for being 75% loyal to the Terrans, but is unknown if you have to have the described percentage in order to get the bonus credits, or if you can have a higher percentage.). They are relatively easy to achieve by simply attacking the opposite faction, or a mission involving fighting a different faction (defending Nivelian freighters from Midorian attackers).