PD stands for power delivery, It allows compatible chargers and devices to negotiate a higher voltage than the standard 5 volts provided by USB, which means things will charge faster.
from a quick search, it looks like the first iPhone to use USB-PD is the 8, so your 6 should not see faster charging when using the PD port.
You should be able to use the PD port in 2 ways.
- to provide higher power to your laptop. So you can plug your charger into the hub's PD port, then the hub will pass the power to your laptop
- to charge external devices at a higher rate. You can plug any PD compatible device (like a phone or iPad) into it, and it will charge faster than if it was plugged into a non-PD port. Assuming the laptop you're using supports PD (which it should if it charges over USB-C). You'll need to plug the charger directly to the laptop.