A die looks like the exact mirror image of the coin it is going to strike, so the words, numbers and portrait busts are set into the surface of a die. If anything fills into those recessed areas, when the die tries to strike a coin, the metal in the coin will not flow into the areas that are filled or clogged. Therefore, the coin will not show any details in that area. They are very common and collectors should be wary of paying a premium for them. A clogged collar die creates a coin with only partially formed reeding on the edge.
Filling can occur on any portion of the coin.
Sometimes coins are found with little or no details at all. This could be a filled die error or it could be a major striking error. They occur when a die cracks under the stress of striking coins. The metal of the coin flows into the crack during striking, forming the line of raised metal. Sometimes a thin die crack can grow into a chunky-looking line as other small pieces of the die fall out along the crack.