D2C stands for direct-to-consumer, which, at the simplest level, means that a brand sells directly to its end customers rather than selling through retailers. For example, H&M has its own online website & physical stores as well.
Although not all eCommerce is direct-to-consumer (if an online shop uses Amazon, for example, this follows the traditional merchant → retailer → consumer pipeline).
The growth of eCommerce has largely fueled the growth of this business model. As more & more consumers shop online, brands that have built solid consumer-facing online stores have received some of this traffic & sales.