What does Judges 6:21 mean?

Given all that has happened to Israel (Judges 6:1–6), and Gideon's own fear (Judges 6:11), he never would have expected to spend his day as he has (Judges 6:12–20). He had no warning that God would appear to him and command the rescue of Israel from the Midianites. In keeping with his deeply-rooted skepticism and insecurity (Judges 6:27, 36–40), Gideon has asked for a sign that this is really the Lord (Judges 6:17). To set up this sign, Gideon prepared a generous meal for the stranger, who asked Gideon to put the meat and the cakes on a rock and to pour the broth over them (Judges 6:18–20).

Now the Angel of the Lord touches the meal with the tip of His staff. Fire leaps up from the rock and burns everything up, even though it was wet from the broth. As soon as this happens, the Man "vanishes:" leaving in a clearly supernatural fashion. This response to Gideon's request for a sign implies several important things. These combined effects are more relevant than a random act of power.

First, Gideon prepared a meal that was the equivalent of an offering, spread it on a rock, and it was consumed by fire. The consumption of the sacrifice was clearly understood as a sign of a deity's acceptance (Leviticus 9:24; 1 Kings 18:38; 2 Chronicles 7:1). Second, the angel of the Lord disappears before Gideon's eyes: there one moment and then gone. This clarifies that what Gideon saw was a manifestation, not a mortal human body.

Gideon now has his evidence that this was really the Lord. Rather than reacting with joy, or courage, he responds with more fear and despair. Now that he knows his Guest was the Lord God, he's overcome with worry that he'll be struck dead for seeing God's face (Exodus 33:18–23).