means The process of joining two pieces of metal by heating them until they melt together and fuse into one solid piece.

from From the verb "weld," which is an altered form of the older English "well" — once meaning to boil or rise up, as a spring of water "wells" forth. By the 16th century "well" had picked up the sense of heating metal until it softens enough to join, and the inserted "d" gave us "weld." The watery root is fitting: both senses turn on something bubbling and rising to the surface, whether spring or molten steel. The English word is related to a family of northern European cognates carrying the same idea of boiling or fusing.