A kickback is a bribe; specifically, a bribe to someone on one side of a larger financial transaction, for facilitating it. As in, vendors A, B, and C are compe

A kickback is a bribe; specifically, a bribe to someone on one side of a larger financial transaction, for facilitating it. As in, vendors A, B, and C are competing for a ten million dollar contract with Walmart to provide some service, so vendor A pays someone at Walmart in a decision making role ten thousand dollars to ensure that they are selected. A small portion of the money that vendor A receives is "kicked back" to the person who facilitated that transaction. Obviously, this is quite corrupt; that person is supposed to be paid by Walmart to best represent their interests, not select the vendor based on who will best financially compensate them personally. The Sopranos part is a reference to a TV show about the Mafia, in which (presumably, I haven't seen it), bribery schemes like this are common.