What does Isaiah 7:3 mean?

The invading armies are in Judah. The kings of Israel and Syria to the north have come to defeat Ahaz, Judah's king. In his place they want to put a puppet king (Isaiah 7:6). They don't want to have to fight Judah when the Assyrians arrive to attack their people (Isaiah 7:1). The Syrians and Israelites have already killed or captured tens of thousands of the people of Judah (2 Chronicles 28). They are making their way to Jerusalem to put the great city under siege. Their goal is to starve the city and its king into submission (Isaiah 7:2).

In this moment, the Lord tells Isaiah to talk to King Ahaz. He tells his prophet where to find the king, a place called "the Washer's Field." It is very likely the king was reviewing Jerusalem's water supply, which was located outside of the city. He would be seeing about securing it so the city would have water during a siege.

Scholars describe the source of this water supply as the Gihon Spring in a valley easy of Jerusalem. Water was brought from the spring through an aqueduct to a reservoir on the southwestern edge of the city. Washer's field was likely a place near the water where laundry was done.

The Lord tells Isaiah to bring his son to this meeting. His son's name is Shear-jashub, which means "a remnant will return." It's unclear if the Lord means for the name of Isaiah's son to suggest something to Ahaz. Isaiah may have named his son after the hope that Judah would have a future after God's judgment comes. That although this judgement would bring near total annihilation, there would still be some people left.