you can take it to mean:Take the sun as your aim (goal,target); even if U miss,U'll at least reach the moon,which is just a bit lower than the sun.
A variant version is:Jump for the sun,at least you land on the moon.
hope this will help U better understand it!
中文里有：“取法乎上仅得乎中,取法乎中者得其下”的说法,异曲同工.还有人据此derive出另一说Shoot for the moon ,if you miss ,you ll land on a star.是不是意义更clear啦?