A monopolistically competitive firm is one that is in competition with many other firms selling identical but not the same products. A monopolist is a business entity that has dominated the market and is in control of a particular product in the market.
So, the above statement means that "The Beatles," were once in competition with other firms selling products that were similar to theirs. Now, however, "The Beatles" are the only suppliers of the product they sell and are in control of that particular market.
