From BR Bullpen
The term Iron Man in baseball generally refers to a player who appears in many consecutive games. Some users of the term may require that the player appear in all innings of each consecutive game.
The term "Iron Man" is occasionally used in other ways. For example, there is a plaque in honor of Dan Brouthers at a field near his birthplace that calls him "Baseball's Iron Man", presumably because he had such a long career.
Chief Zimmer was called an "iron man" because he caught so many games at a time when catchers took a beating from stray balls and usually did not catch every day.