Omnichannel retail is a retailer's way of joining its stores, its app and its delivery service into one system, so that the same store serves customers who walk in and also packs orders placed on the app. "Omni" means "all", and a channel is any way of selling, such as a store, a website, an app or a marketplace. The general idea is explained on the page on omnichannel.
A retailer is a company that buys goods from their makers, owns them and sells them to you. A marketplace is different: it lets other sellers list their own goods, and never owns them. The page on the marketplace vs inventory model explains the difference. A retailer with hundreds of stores already has goods on shelves close to almost every customer. Omnichannel retail uses those shelves for online orders as well.
The hard part is the stock count. If the app shows that 20 packets of a biscuit are available, but walk-in customers bought the last ones an hour ago, the app order fails. So every sale at the billing counter must reduce the same count that the app reads. The counter runs on retail POS software, which reports each sale at once. The idea that every channel reads one stock count and one set of orders is called unified commerce.
Here is a typical case. A supermarket in Ahmedabad is open from 8 am to 10 pm. At 7:40 pm, a customer two kilometres away orders 12 items on the chain's app. The retailer's order management system (OMS) checks which nearby store has all 12 items and sends the order there. An employee in the store gets the list on a handheld phone, sorted by aisle. She picks the items from the same shelves as the shoppers around her, and packs them in about six minutes. A rider collects the bag, and the customer has it within 30 minutes. The stock count drops by 12 items at once, so the app and the counter both show the same numbers.
Some retailers go further and turn part of a store, or a whole building, into a dark store. A dark store is a small warehouse that customers never enter, from which orders are delivered in minutes. This competes directly with the quick commerce apps.
Joining the channels also joins the customer's record. A customer who earns points at the counter can spend them on the app, because one loyalty programme records both. India's largest retailers are building this at the scale of thousands of stores, and many of them also list the same goods on marketplaces.