What is a weld symbol?

A weld symbol is the shape drawn on a Print to tell the welder: how to prepare a welding joint, what process can be used to weld (mig, tig, arc, etc) what filler rod to use, how wide and long the weld should be, how to deal with the weld after your done (grind flush, brush clean, or nothing) and a million other things....

Its a tool that design engineers use to communicate what the weld needs to be to meet their design specifications.