What is meant by the Pirate term "hang him from the yardarm?" What is a yardarm?

A yardarm is part of a spar on a mast from which sails are set and a person could be hung from there.Yardarms
The outermost tips of the yard: outboard from the attachments for the lifts.

https://en.wikipedia.org/wiki/Yard_%28sailing%29

The yardarm is the horizontal spar of a square-rigged vessel.
"Execution by hanging at the yardarm was the normal punishment for mutiny in the fleet. The last execution was carried out in 1860 (during the Second Chinese War) on a marine who attempted to murder his captain. Yardarm execution as carried out in the navy is well described by Nordoff and Hall in Mutiny on the Bounty. As a capital punishment it was by no means instantaneous as is said to be with the case with our modern Canadian practice. The prisoner's hands and feet were tied, and with the noose about his neck a dozen or so men, usually boats' bowmen (the worst scoundrels in the ship) manned the whip and hoisted him to the block of an upper yard, to die there by slow strangulation."
http://readyayeready.com/tradition/customs-of-the-navy/6-laws-of-the-sea-and-punishments.htm