I am assuming you are talking about the part where Marley's ghost is talking to Scrooge and Scrooge tells him he was always "a good man of business."
At that point, Marley's ghost says:
'Business!' cried the Ghost, wringing its hands again. 'Mankind was my business. The common welfare was my business; charity, mercy, forbearance, and benevolence, were, all, my business. The dealings of my trade were but a drop of water in the comprehensive ocean of my business!'
What Marley is saying is that his business should have been people. He should have cared more about them and less about making money. He says that his and Scrooge's firm was really just a little thing compared to what he should have been concerned about.
So the word seems to mean what Marley should have cared about as opposed to the company that he and Scrooge owned which was what he really did care about while he was alive.
In stave l, Jacob Marley's spirit uses the word "business" in reply to Ebenezer Scrooge's statement that Marley had always been "a good man of business" during his lifetime. The spirit is clearly upset by Scrooge's use of the term and responds vehemently.
What Scrooge means is that Marley had been successful in running an enterprise in partnership with him. The two men had a shared interest in a successful venture. The partnership ceased with Marley's death seven years before. Scrooge now runs the business but has not removed Marley's name from the business title.
Marley's present contention is that the 'business' Scrooge speaks of is not what Marley's chief interest should have been in his lifetime. He states that Mankind was his business. In other words, he should have taken a greater interest in other people. He rants that "the common welfare was my business," which suggests the interests of all people was the business he should have focused on. He states that he should have been charitable, merciful, patient, and kind. He asserts that whatever trade he had been performing was a minuscule task compared to the enormous duty he was supposed to have performed.
The ghost's contrasting interpretation of the word becomes obvious when one looks at the context of its situation. When Marley died, he was damned to eternal misery, chained to the objects he had spent so much of his time on in life, to the exclusion of everything else. He had been completely immersed in running a business, his focal point being profit. As such, he had no time for anyone else and had become selfish, cold, and uncaring. He was relentlessly greedy and stingy, had no compassion, and refused to extend any generosity to anyone. His punishment for such ruthless indifference was the severe sentence mentioned above.
It is for this reason that Marley's ghost so passionately states that his business during life should have been to show greater compassion and be more involved with the rest of humanity.
Marley has decided to visit his old friend and partner to warn him against the dangers of selfish, careless, and inconsiderate behavior, character traits that Scrooge has, too. He is concerned about Scrooge and has come to admonish him and provide him guidance to avoid such an ignominious and desperately depressing destiny.