The quote really speaks for itself.
Tom: Do we give your son-in-law something important?
Vito: Never. Give him a living, but never discuss the family business with him.
What Tom is asking is, should they give Carlo a job high up in the Corleone organisation and Vito is say "No".
Vito wants Carlo to be given a low level role in the organisation but nothing where he could learn anything important.
Equally, Tom is instructed to make sure that Carlo is not given any information about the higher workings of the Corleone organisation even though he is a member of the actual family by marriage.
The novel makes this clearer...
“Your new son-in-law. Do we give him something important, inside the Family?”
He was surprised at the vehemence of the Don’s answer. “Never.” The Don hit the desk with the flat of his hand. “Never. Give him something to earn his living, a good living. But never let him know the Family’s business. Tell the others, Sonny, Fredo, Clemenza."
The Godfather - Mario Puzo