It's a monthly "surcharge" Sprint charges for using smartphones like the iPhone on their network. They claim it's necessary because smartphone users use more data than non smartphone users. It's in the fine print but doesn't appear anywhere in their advertising until you get to the check-out page to pay for phone & service.
Buying an iPhone 6? Sprint's $80/mo for Unlimited Voice, Text, & Data sounds like an excellent deal until they add $30/mo Premium Data charge during check-out. Do the math and you're really paying $110/mo.