Being grounded is knowing your right to be here, now, to have your body and to be your body. Grounding is the root from which all else grows.
Imagine a large tree with roots that sink deep into the earth. It is strong, but supple, enabling it to withstand even intense storms. This is the way of the tree, and it holds a lesson for us: Root your spirit and consciousness in your cells and organs. Root your physical body in good food, exercise, and love for nature. Learn to love your body. This is the way to build a strong spirituality.
We become ungrounded for many reasons. Perhaps we have been abused physically or sexually and have learned to disconnect from our bodies. Perhaps we were denied basic physical needs as children, and so have been taught to deny our own bodies. Perhaps we are focusing all our spiritual work on the upper chakras, hoping for psychic ability, whilst neglecting a block that is preventing energy flowing downwards. In these cases there will be excess energy in the upper chakras, over-thinking, tension and headaches.
Energetic practices that aid grounding include giving energy to the lower chakras and the lower body. This is why we finish Reiki treatments at the feet. Sweeping movements in a downwards direction through the energy field (a few inches away from the body) can also help. Finally, if a large amount of energy goes into the head, try to ensure this is balanced with energy into the lower body too.