A 2-iron is a type of golf club. With the least loft of any iron other than the 1-iron, the 2-iron is used to hit long, low shots.
The 2-iron used to be much more common, even ten years ago, but nowadays most players don't even carry a 2-iron in their bag, preferring to carry additional woods and going only with the irons from 3 to pitching wedge.
This is because hitting a long, accurate shot with a 2-iron takes a considerable amount of skill, so most players find they are better off hitting a 3-wood, a hybrid wood, or taking two shots to cover the distance.
Nevertheless, in the hands of a very skilled player, the 2-iron can still be a useful weapon, especially for hitting low, arcing shots around trees or other obstacles, or on windy days when it is advantageous to keep drives lower to the ground.