S4 Ep63 Looking at Isaiah 6:8 (“Here am I, send me") in Its Context

We visit another oft-quoted verse (often when preachers are calling missionaries or guilting parishioners into evangelistic duty), Isaiah 6:8.

Also I heard the voice of the Lord saying, “Whom shall I send, and who will go for us?" Then I said, “Here am I. Send me.”

But what message was Isaiah sent with? And to whom was he sent and why?