The passage is actually Psalms 140, not 139. Here is a summary of Psalms 140:
David asks the Lord to rescue him from evil men who plot against him and seek to harm him with their words and actions. He asks for protection from their snares and stumbling blocks, and for the Lord to hear his prayer and be his salvation. David trusts that the Lord will bring justice to the wicked and vindication to the poor, and that the righteous will be guided by God's presence.
1 Unto the end. A Psalm of David 2 Rescue me, O Lord, from the evil man. Rescue me from the iniquitous leader 3 Those who have devised iniquities in their hearts: all day long they constructed conflicts 4 They have sharpened their tongues like a serpent. The venom of asps is under their lips 5 Preserve me, O Lord, from the hand of the sinner, and rescue me from men of iniquity. They have decided to supplant my steps 6 The arrogant have hidden a snare for me. And they have stretched out cords for a snare. They have placed a stumbling block for me near the road 7 I said to the Lord: You are my God. O Lord, heed the voice of my supplication 8 Lord, O Lord, the strength of my salvation: you have overshadowed my head in the day of war 9 O Lord, do not hand me over to the sinner by my desire. They have plotted against me. Do not abandon me, lest they should triumph 10 The head of those who encompass me, the labor of their lips, will overwhelm them 11 Burning coals will fall upon them. You will cast them down into the fire, into miseries that they will not be able to withstand 12 A talkative man will not be guided aright upon the earth. Evils will drag the unjust man unto utter ruin 13 I know that the Lord will accomplish justice for the needy and vindication for the poor 14 So then, truly, the just will confess your name, and the upright will dwell with your countenance