Serenity Caldwell, writing for iMore:

Instead of routing your voice packets through your carrier’s closest cell tower, those packets get tunneled through the Internet to a controller used by your cell company, which then bounces them across the network to whomever you’re speaking. The net result is that you’re talking, but you’re not actually using a cell tower to do it.

If your carrier supports Wi-Fi calling, it’s a great way to get clearer, crisper calls when chatting with all your friends—not just those on an iPhone or Mac.

I’d also suggest check out Rene Ritchie’s tip on adjusting 3D Touch sensitivity.