The stock trigger of a phantom is a stamped metal piece that is somewhat thin and uncomfortable. The triger shoe slides over the trigger, making the part of the trigger that you grip much wider and more comfortable.
They generally do not come stock on the phantom, but you can get a shoe for less then $5 usually from any number of vendors.
Here is the shoe itself...
Here is how a trigger shoe fits on a trigger (this one is not on a phantom though)
Here is one installed on a phantom trigger...
Get the picture? It took me a while to understand as well, but it made perfect sense once I actually picked up my phantom and saw it there.