Power Delivery (PD) is a specification for handling higher power and allows a range of devices to charge quickly over a USB connection. It operates by facilitating a conversation between two devices to negotiate a power contract so they can determine how much power can be pulled from the charger. Power Delivery starts at the 5V setting and is configurable up to 20V. Using a standard USB-C cable, it can handle up to 60W, and will go up to 100W using a designated EMCA cable.
As the USB-C connectors are designed hand-in-hand with Power Delivery, it is able to handle high levels of power for fastest possible charging without the risk of damaging the circuitry, as the USB Power Delivery chipset in the connector can automatically identify the connected device and deliver the exact power required and not anything more.
Apple introduced USB PD charging with iOS devices with the launch of the 2015 iPad Pro 12.9" and with OS X laptops in the MacBook Pro as of 2016. The iPhone 8, the iPhone X and XS/Max can rapid charge with USB PD using any USB PD charging accessory -- you don't have to use Apple's OEM USB-C 29W or its 61W power adapters.