What is Gideon's ephod, and why did he build it? In the story of Gideon (Judges 6-8), near the end it talks about Gideon building an ephod out of the gold earrings from the spoils of the battle against the Midianites. What is an ephod, and why did gideon build one. Is it significant?
in early books of the bible, the ephod was a special piece of clothing that was made for Aaron the High priest to wear when he served the Lord (Exodus 28:5)


  “They shall take the gold, blue, purple, and scarlet thread, and the fine linen, 6 and they shall make the ephod of gold, blue, purple, and scarlet thread, and fine woven linen, artistically worked. 7 


When I started to research this question, I realised that everyone pretty much agrees the ephod in Judges was not a clothing, like I had always thought!

So I cant tell you exactly what it was physically, but I can answer the 2nd part of your question and explain some significance.

It says in Judges 8:24


  4 Then Gideon said to them, “I would like to make a request of you, that each of you would give me the earrings from his plunder.” For they had golden earrings, because they were Ishmaelites.


This is extremely familiar scene. Think back to exodus as Moses goes up the mountain to get the 10 commandments. The people think he has been gone to long, and they ask Aaron the priest to make them an idol!

Exodus 32


  Now when the people saw that Moses delayed coming down from the mountain, the people gathered together to Aaron, and said to him, “Come, make us gods that shall go before us; for as for this Moses, the man who brought us up out of the land of Egypt, we do not know what has become of him.”
  2 And Aaron said to them, “Break off the golden earrings which are in the ears of your wives, your sons, and your daughters, and bring them to me.” 3 So all the people broke off the golden earrings which were in their ears, and brought them to Aaron. 4 And he received the gold from their hand, and he fashioned it with an engraving tool, and made a molded calf.


So the significance of the ephod in Judges, is that once again, God's people do not trust in him or give him glory for the things he has done for them, but they go astray and break his commandments by making for themselves idols and worshipping them.

Idol worship is a very very prominent theme of the Old Testament. and it was a problem the Israelites continually faced.