Collections are groupings of products you can set it up for the customers. They can browse the products by a certain category. For example, a makeup merchant might set up a collection of lipsticks.
You can also set up seasonal collections like summer products or collections of products that are on sale. You can show your customer a list of all your collections, so they can easily find the selection of products they're interested in.
Collection Field is also provided in the field mapping section for Add feed only in Wix.