The label is something you make up: a few letters and numbers. The proxy network remembers which address it gave to that label and keeps using it. Change the label and you get a different address; reuse it and you get the same one back, for as long as the session lasts.
This makes it easy to run several separate visitors at once. Ten labels are ten addresses. Give each browser profile, account or worker its own label and never share one between two of them. On PiratProxies the label is eight characters, as in USERNAME-country-us-time-600-session-a1b2c3d4, and the generator on each plan page can write a whole list of them.
Read next: One proxy per account