The short answer
A cloud marketplace is a catalogue inside AWS, Azure or Google Cloud where a customer buys your software through the procurement they already use for cloud, often drawing down a spending commitment they have already made to that provider. That last part is the real magic: money already promised to AWS can be spent on you, which turns a budget fight into a procurement formality. Listing is free and mostly paperwork, and the provider takes a percentage on transactions, lower on private offers if you are in a co-sell program. It matters enormously if your buyers are deep in one cloud, and barely at all if you sell to Indian SMBs paying monthly by card.