Salal (H5539) carries the double meaning of heaping up (building a highway or mound) and, in some contexts, the contempt of treating something as light or worthless. Most commonly it means 'to cast up a highway' β the preparation of a royal road through rough terrain, commanding mountains to be leveled for the king's approach.
Isaiah 57:14 and 62:10 use salal for the eschatological highway of return: 'Build up, build up, prepare the road! Remove the obstacles out of the way of my people!' This is the precursor to John the Baptist's role (Isaiah 40:3 β related concept) and the final gathering of Israel. The theological image: God commands barriers to be cleared so His people can walk freely back to Him.