‘Dan tian’ can be said to be the major “reservoir” and “transistor” for the qi in the body. The major dan tian is called the Lower Dan Tian. For men it is three or four fingers below the navel. For women it is just below the navel because the womb pushes it up a little higher.
There are actually many dan tians. Most people talk about three in particular: the Lower Dan Tian, Middle Dan Tian and Upper Dan Tian. The Lower Dan Tian is considered the major one which is the lower circle in the diagram below. The Lower Dan Tian is the center or core of the body; therefore the other dan tians build off of it. (See the Jing Qi Shen section of this website.) So strengthening the Lower Dan Tian is very important not only because it is a main source of energy, but it is also the "core" of the body. The best and easiest way to strengthen it is to breathe by using the lower abdomen. When doing a "core workout", one is working the center, the Lower Dan Tian.