This is what he showed me: The Lord was standing by a wall that had been built true to plumb, with a plumb line in his hand. And the LORD asked me, 'What do you see, Amos?' 'A plumb line,' I replied. Then the Lord said, 'Look, I am setting a plumb line among my people Israel; I will spare them no longer.
I guess this means that if we don't straighten up he will have no more mercy on us. Again, since God does not change, it is Him showing us that he won't put up with our antics forever. We make a choice to get in line or fall out. I want to be "plumb" with our Lord.
E-How's, Shelley Shaub said this:
What Is a Plumb Bob?
"A plumb bob is a humble tool consisting of a weight at the end of a string. "Plumb" comes from the Latin term "plumbum" for the element lead, indicating a lead slug tied to a string. One end of the string is affixed at a point above the floor or ground, and the weight is allowed to hang free until it stops moving. This shows the user "true vertical." With a plumb bob, anyone can measure or draw a line parallel to Earth's gravity. Plumb bobs often have carefully machined brass or bronze weights with points on the end and cotton or water-resistant nylon string. The point represents the spot directly below the origin of the string. Aside from modern plumb bobs using newer manufactured materials, the plumb bob has not changed in millennia"
I think it interesting that we know that the plumb bob has not changed in a millennia. How God showed Amos a plumb line and Amos knew what it was and what it was for. Therefore the symbolism stands perfectly straight.