Distributors sell investment products to people, often the retail investor, independent of AMCs but drawing a fee from them.
The AMCs pay the distributors for their financial products we buy from the latter, in the form of a commission that is the percentage of the invested amount. For recurring transactions such as a SIP with a distributor, the distributor gets a trailing commission on the invested amount, again as a percentage of the amount invested.
Due to inefficiencies in the industry, distributors often display their biases despite being a third-party player. They push products and AMCs which give them more commissions rather than what is best suited for the investor.