The blend of marketing, sales, communications, and technical digital product knowledge. Product marketers are responsible for things such as product brand strategy, product hierarchy, product packaging, and positioning.
Product marketers are like a realtor selling your home: They have comprehensive knowledge of every detail about the house, but they also know how to explain those details in a way that appeals to specific buyers wants and needs. When a realtor is showing you a house, they don't tell you how many outlets the house has because those aren't the details you care about. They know your priorities and will explain the house to you in a way that resonates.
Product marketers act as the bridge between technical product knowledge and how you market and sell the product. Without product marketing, you run the risk of buyer confusion about what your product does and what differentiates it from competitors.
Digital products need product marketing to make sense of the features, benefits, and value provided by that product.