Other forms: threw down the gauntlet; throwing down the gauntlet; throws down the gauntlet
To throw down the gauntlet is to challenge or dare someone. Think about historical movies where they're challenging each other to duels all the time, and you'll get the idea.
The expression throw down the gauntlet comes from the customs of medieval Europe. When a knight wanted to challenge an opponent, it was done by taking the gauntlet, a heavy, armored glove, and throwing it at the opponent's feet. As armor became less common, ordinary gloves became what you'd throw down when you wanted to duel someone, but the expression didn't change.