Zerach as a verb means to rise or shine — particularly used of the sun rising or of light breaking forth. As a proper noun, it is the name of several biblical figures, most notably Zerah, the twin son of Judah (Gen 38:30), whose scarlet thread became a prophetic symbol, and Zerah the Cushite who brought an enormous army against Asa of Judah (2 Chr 14:9). The word is cognate with zarach, to rise/shine.
The rising and shining of zerach carries rich messianic associations. Malachi 4:2 uses the closely related noun shemesh tzedaqah — the 'sun of righteousness' — that will rise with healing in its wings. The dawn-imagery throughout Scripture points to God as the source of all light breaking into darkness. The scarlet thread tied to Zerah's wrist in Genesis 38 became a redemptive symbol — a thread of promise in a dark story, foreshadowing the scarlet line of Rahab and ultimately the blood of the Lamb.