Isaiah 8:18. "Behold, I and the children whom the Lord has given me are signs and portents in Israel from the Lord of hosts, who dwells on Mount Zion."

God is telling Isaiah to say these words to the people of Judah. He wants them to see Isaiah and his two sons as a signs, as examples of what it means to trust God, if not now, then when they remember back to this time. Prior to this, God warns Isaiah not to walk in the way of the people, saying "do not call conspiracy all that this people call conspiracy, and do not fear what they fear, nor be in dread." (Isaiah 8:11-12). It seems the people of Judah are feeling the same kind of unease that we are feeling today and, just like today, they are casting about for things to blame for this feeling. They are fearful and in dread, seeing conspiracies everywhere, unable to think rationally. One reason for this feeling, apart from their own misdeeds, was the threat of two kingdoms, Syria and Israel, poised to come against them. God told Isaiah to tell King Ahaz not to worry about these threats, that He would protect Judah, but Ahaz would not listen.

Ahaz's  distress over this looming threat was affecting his people, as the king goes, so goes the nation, after all. But Isaiah knew not to be afraid, because God told him not to be afraid, so he and his sons did not feel the same fear the people of Judah did. They were living their lives without the fear and anxiety that was affecting the people of Judah, so they became signs, not only by overt prophecy, but also just by living differently from those around them. They were living without fear, and God hoped the people would notice and remember that God wanted to protect His people. If they didn't notice now, they would at least look back and remember the man who conversed with God had not been afraid, nor were his children. Perhaps, in the future, they will listen to such men, rather than sacrificing everything to their fear and stubbornness.