Marley said that his chain was "forged in life". This means that all of the bad things he did in the course of his life came back to haunt him, in the form of this chain, which will cause eternal agony for him while he walks around Earth in ghost form.
Every ghost has a chain of some sort. An example given in the book is when Scrooge recognized a banker that he knew, flying around with a gigantic iron safe chained to his leg. So the banker must have cheated people while he was living, and now the safe is a constant reminder, and shows his guilt.
Jacob Marley's ghost had chains to symbolize the burden of his sins and greed from his life. The chains represent the heavy weight of his wrongdoings that he carries with him in the afterlife as a form of punishment. This imagery is used to convey the message of redemption and the consequences of one's actions in "A Christmas Carol" by Charles Dickens.
We still use it in our country (especially in traditional families). The mouth of the deceased will open some time after death (if the body is not "treated" by a mortician, of course), so we bind it with a piece of cloth in order to keep it shut. Not nice, isn't it?
I can't be too sure, but if I'm right then it's because it's showing his greed, and because he refused to let go off his money he got stuck carrying chains. Basically, in the simple terms, the chains represent his greed, hence why he was warning Ebenezer.
It is a chain that he states he forged in life dect out with case iron case boxes, keys safes ledgers and money bags all representing what was important to him - he wears it as a self imposed punishment for not caring about mankind
Dickens describes them as being made "of cash-boxes, keys, padlocks, ledgers, deeds, and heavy purses wrought in steel."
Jacob Marley, the chains represented the sins of his life
The ghost with chains in "A Christmas Carol" is Jacob Marley, who appears to Ebenezer Scrooge as a tormented spirit wearing heavy chains. Marley's ghost serves as a warning to Scrooge about the consequences of his selfish and greedy ways.
It was Jacob Marley Scrooges former business partner
He said they were the chains he forged in life, link by link, in his misdeeds with other people.
Marley's ghost, the first of four, visits Ebenezer Scrooge.
Jacob and Robert Marley :)
A toothpick. Scrooge challenged Marley's Ghost with the thought that swallowing the toothpick would make the ghost disappear. When Marley shrieked in response, Scrooge finally came around to believing Marley was there.
The Ghost of Christmas Past
The ghost of Jacob Marley, Scrooge's former business partner, was responsible for the sound of dragging chains that Scrooge heard. Marley's spirit visited Scrooge to warn him about the consequences of his greedy and selfish ways.
Goofy portrays Jacob Marley in Mickey's Christmas Carol, he comes to tell Scrooge (Scrooge McDuck) that three spirits will visit him soon, Goofy has a ghost-like appearance and is in long chains.
Ebenezer Scrooge's partner in "A Christmas Carol" is Jacob Marley. He appears as a ghost wearing chains and warns Scrooge about the consequences of his selfish ways.
A Christmas Carol