Prop Control Utility:

Lock: This button sets keys at the current frame on multiple custom attributes, to effectively lock the appropriate hand to the Prop Control object. When a hand is locked to the PC object, both the position of the Arm IK Control and the rotation of the Hand bone are locked.

Release: Opposite of the Lock button. When a hand is locked, and you want to release it for any reason (character drops the object, scratches his head, etc) click release and from then forwards the hand will be controlled as normal.

Offset Option: The "offset" is how the system knows where to lock the hand. It contains position and rotation information - specifically, the difference in position and rotation between the PC object and the Arm IK Control/Hand bone in question. The options are explained below.

Locking:

Key New: This will key new offsets based on the current position of and rotation of the hand. You would use after you have posed your character holding the prop for the first time.

Keep Old: This uses the offets from the last Lock key you set. You would use this if you have released a hand, for example to do a reload animation, and then you want to animate it going back to the previous locked pose. The position and rotation will correctly interpolate from the last key you set (while released) back to the appropriate prop-holding pose.

Releasing:

Key New: This will keep the hand's position and rotation the same after releasing. This is almost always what you will want to do when releasing.

Keep Old: This will release the hand and send it back to the position/rotation it was at when you last locked it (in world coordinates). I have no idea why you would want to do this, but the option is there just in case. :)

Key Prop_Control?: Selects whether you want to also set position/rotation keys for the Prop Control Object along with the lock and release keys. I recommend keeping this checked, but there might be times when you want to Lock or Release a hand while the PC object is on an arc that you don't want to disrupt with a key.