The Skull 'n' Bones, more commonly known as the Jolly Roger, was first used in early 1710-1720s by three famous pirate captains - "Black Sam" Bellamy, Edward England and John Taylor. This was the Golden Age of Pirates.
 
This symbol is also used as memento mori (Latin for "Remember you must die") on tombstones and entrances to cemeteries.
 
In modern times it symbolizes any form of resistance against authorities.