Albert Barnes Commentary Psalms 139:5

"Thou hast beset me behind and before, And laid thy hand upon me." — Psalms 139:5 (ASV)

Thou hast beset me behind and before - The word rendered “beset”—צור tsûr—means properly to press, to press upon, to compress. It commonly refers to the siege of a city or the pressing on of troops in war; it then comes to mean to besiege, hem in, or closely surround, so that there is no way of escape. This is the idea here: God was on every side of him, so he could not escape in any direction. He was like a garrison besieged in a city, with no means of escape. There is a transition here (not an unnatural one) from the idea of God's omniscience to that of His omnipresence, and the remarks that follow mainly refer to the latter.

And laid thine hand upon me - That is, if I try to escape in any direction, I find your hand laid upon me there. Escape is impossible.