What does the skull head stands for?

Question

what i'am saying is the skull with the face the ones that's on the pirate

Answer 1

I believe the origional skull and cross bones flag  or black flag was ment as a warning to other ships of a ship that carried plague.  

Now that symbol is used to represent poisonous substances.  

Answer 2

"The Jolly Roger" is used these days on labels to indicate that the contents are poisonous to either humans or animals.

Answer 3

toxic