‘Oh send out your light and your truth,
Let them lead me,
Let them bring me to your holy hill,
And to your tabernacles.
Then will I go to the altar of God,
To God my exceeding joy,
And on the harp will I praise you,
O God, my God.'
‘Send out Your light and Your truth.' Perhaps he has in mind here the pillar of light that had led God's people from Egypt, and the light and truth revealed at Sinai. It was by these manifestations of God that Israel had been delivered. So now he wants God to act in the same way on his behalf, delivering him and leading him back to God's holy hill and to His tabernacles. (The plural for tabernacles may suggest the time of David when there were two tabernacles, one in Hebron which was the main centre of worship, and one in Jerusalem that held the Ark of the Covenant of YHWH. Or it may simply have in mind the Temple as the dwelling place of God seen in plural majesty). Then he will again be able to go to the altar of God, to God Who is his great joy, and will be able to praise him on the harp because He is God his God.
Alternately the thought is that in the end God's light and truth will always prevail, so that it must result in the deliverance of His people. (Possibly also he sees the armies of Israel as representing God's light and truth). But the point is that once the God of light and truth comes to deliver him nothing will be able to prevent his release, for light and truth must always prevail. It is a salutary reminder that our salvation also is totally due to the coming of One Who was the Light and the Truth (John 8:12; John 14:6).