Dumb as a Light Post

They said I'm as dumb as a light post but what does that even mean? I really don't get it is that supposed to be mean? I don't understand should I be offended? I honestly don't know can this be comprehended? Are light posts dumb now because I didn't know they could talk or think. But at least they light up the world and make our pupils dilate and shrink. But seriously, if you use the insult 'dumb as a light post' I think we both know who really is 'dumb as a light post'.