Hand Canceling by the USPS is just where a postal employee uses a stamper to put a cancel stamp over the stamp that is on the envelope or on the post card or other mail.
When you put a stamp on the envelope or other mail the mail goes through a machine that then cancels out the stamp which makes it impossible to reuse the stamp.
Some people have the USPS postal employees hand cancel the stamp so that the machine doesn't ruin the letter or the postcard.
You have to ask the postal employees to hand cancel the letter or other piece of mail or the letter or mail will go through a machine in a sorting facility where it gets machine canceled.