"The Angel of the Lord came, and He sat under the oak that was in Ophrah, which belonged to Joash, the Abiezrite. His son Gideon was threshing wheat in the wine vat in order to hide it from the Midianites. Then the Angel of the Lord appeared to him and said: 'The Lord is with you, mighty warrior.'"
God sent His personal representative, the Angel of the Lord, to Gideon at Ophrah. Because He was the Angel of the Lord, He spoke with God's full authority.
Things were pretty bleak in Israel at this time. Whenever the Israelites planted crops, large bands of nomadic raiders would attack and plunder them leaving nothing for Israel to eat.
Because of the threat from the Midianites, Gideon was threshing wheat in a wine vat, a shallow depression in the rock. This was a safer location than the more exposed threshing floor.
The title given to Gideon by the angel--mighty warrior--is in sharp contrast to Gideon's reticence and overly cautious nature. Time and again Gideon required reassurance from God that he was the one called to accomplish God's will or that he was doing what God had called him to do.