Grounding and Centering is a technique for balancing the energy within your body.
When you feel run down, you can draw more energy in, feeling recharged. When you have an excess of energy, such as nights when you just can't seem to fall asleep, you can release energy so that you no longfer feel jittery or like you have to do something.
Normally, Grounding & Centering is done in ritual after you do a working, be that Magick, or any other form of raising energy.
If you don't Ground & Center, you may feel drained, or antsy because your energy level is too low or too high.
You Ground & Center at the same time. You place your hands on the ground, or slap the ground, feeling the energy move from your body, through your hands to the ground, and back.
It's important to realize the energy flows both directions. If you have an excess of energy, it will flow from you, if you are deprived of energy, it will flow to you.