What does it mean to "wait upon the Lord"?

Please explain what is meant by “waiting upon the Lord,” as in Isaiah 40:31?

Several Hebrew words are translated, “wait,” in the Old Testament. The root used here for “wait upon” is different from that, say in Psalm 62:5, and elsewhere, where the R.V. Margin has “to be silent” (“dämam”—properly “to be dumb”). Here the word is “qãvãh, of which the primary meaning is “to be strong,” and then “to await” (the idea being perhaps that waiting God’s answer does need a certain strength of purpose or endurance). The two roots are found together in Psalm 62:5, “My soul wait (dãmam—be dumb) thou only upon God,  for my expectation (qãvãh root) is from Him.” Isa. xl. 31 then means that they that come to the Lord and tell Him their needs, and are willing to wait His time for an answer, “shall renew their strength.” Waiting upon God gets the answer some day, and strengthens the soul to-day. Therefore “wait (qavah) on thy God continually” (Hosea 12:6).