The psalmist seems to have been comparing himself to the stone that the builders (his adversaries) had rejected, in view of the preceding context (cf. Psalm 118:18). The imagery is common. Whenever builders construct a stone building they discard some stones because they do not fit. The writer had felt discarded like one of these stones, but God had restored him to usefulness and given him a position of prominence in God"s work. "Corner stone" (NASB) is more accurate than "capstone" (NIV). The cornerstone of a large building was the largest and or most important stone in the foundation. All the other foundation stones were laid and aligned in reference to this key stone. Only God could have done this ( Psalm 118:23). The day of his restoration was obviously one God had brought to pass. Consequently the writer called on everyone to rejoice with him in it. [source][source][source]
There are many New Testament references to the stone of Psalm 118:22. The Lord Jesus applied it to Himself ( Matthew 21:42; Mark 12:10-11; Luke 20:17). Peter and Paul also applied it to Jesus ( Acts 4:11; Ephesians 2:20; 1 Peter 2:6-8). God"s amazing resurrection of His rejected Son to the place of supreme universal authority is marvelous to say the least. The day of His resurrection is the greatest day the Lord ever made. It is indeed the basis for the Christian"s joy and rejoicing. [1][source]