As the name suggests, a marketplace is a digital « market square ». It is also a business model.
Picture a market square where sellers of food products gather, selling more or less the same goods but each working independently, paying a small contribution to the local town hall for the right to occupy a stall.
Digitise that square as a website and replace the fruit & vegetable stalls with sellers of any kind of product. Each stall pays a cut of its revenue to the marketplace that hosts it - and there you have a digital marketplace.
The French marketplace CDiscount is a good example. They build the platform and the complex technology around it, then each brand chooses to offer its products, paying a commission on each sale.
First adopted by retail and consumer-to-consumer sales (eBay, Amazon, Rakuten…), the marketplace model quickly spread to any type of service (rides for Blablacar, accommodation for Airbnb, and so on).