How does priority mail work?

so im new the U.S. let alone sending priority packages through USPS...i have a box and i have my item i want to send and on the top right corner it says priority postage required..is there a special stamp i'm suppose to use or will a regular stamp not do..after i figure that out do i just drop it off into the mailbox or am i suppose to hand it to somebody who works at the post office? is it like any regular mail where you can just