Will when talking about a sword says, “the tang is nearly the full width of the blade.” What does that mean?

The tang is a part of the sword, that is connected to the blade, and located directly below the hilt. The rule of thumb for the tang is, the thicker it is, the sturdier and stronger the blade, however…the thicker it is, the (slightly) more weight you are adding to the blade, and you have to be careful so that it is kept balanced. See this diagram to see where the tang is located.