A channel in an eCommerce world means every place that you want your data about products to appear. So in other words, when you're creating a channel, it means that you're pushing the data out of the PIM to different locations - they can be either online or offline locations.
Online locations can include other stores that are managed by different markets, or maybe some kind of marketplaces like eBay, Amazon, Etsy. Sometimes you'd like your products to appear in rating sites - you also need to present there your most updated products. And mostly, your store also has a mobile app where your sales are rocketing so you want to keep it up-to-date.
Offline locations may include point of sale, or any print catalog. You also need to take care of that part of your audience and PIM makes sure that through these channels you'll have always up-to-date products.