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.