What is "being grounded"?
Grounding is where every cell in the body is relaxed and has fallen under the force of gravity and is no longer fighting against gravity or using the tension of splinting muscle groups or effort to force or uphold the bodies upright posture.
Being grounded means that you have created, or become a clear, unobstructed physical conduit where the weight of your body (and any incoming outside forces - including gravity) transfer through into the ground. When grounded, you should feel very mentally relaxed and heavy in your body, yet effortlessly suspended and surprisingly agile.
In a grounded state, it is as if the earth itself has risen through your legs, supporting your body from within without required effort from your musculoskeletal system to assist in suspension. In the internal martial arts and Taijiquan classics with this concept is expressed by the sentiment that "how does an opponent defeat you when he is fighting the earth, itself?".
Grounding also applies to seated and lying qigong postures as much as it does to upright postures such as standing and walking. This means that we should always be allowing our body to be ground. Even sitting on a seat at a table with your elbows resting on the tabletop your weight should ground through your sitting bones and your arms should be grounding onto the table and not suspected by muscular effort or tension.
It can be very advantageous to over time become more and more aware of this principle in your daily life. Becoming aware and focusing your attention on grounding throughout your daily activities is a great way to anchor your mind and sink you qi into dantian, which will limit your qi floating up and collecting in the chest. When qi accumulates and pockets in the chest, it can causes tension in the neck and shoulders, vex the heart and/or cloud the lungs causing anxiety, depression, headaches and other disease states.
Understand that the more you practice grounding, the more you will be able to maintain and embody this principle. So when sitting watching TV, washing the dishes, waiting in line at the shops or brushing your teeth, using these times to practice the art of grounding is a useful way to start filling those test tubes and infusing the concepts and experience of grounding into your mind and body.
Many Qigong and Tai Chi schools take grounding to indicate that someone is immovable. They take up wide stances and have competitions to see who can move or push the Master over. And while this is a skill in its own right and has its own circumstantial applications, it is not necessarily indicative of what a grounded state is actually about. Undoubtedly, yes, these people are grounded when playing this game and drills like this are useful in for students to discover, learn about and cultivate grounding whilst building correct internal tension. But grounding is more about the state of the mind and body than a parlour trick used to inflate your guru status by presenting a specific and natural physical structure as "qi magic".
And while these games are an example of the principles and structure of grounding, it is not necessary to take wide stances and become immovable to be grounded. Grounding simply is the ability to let one's entire mass to sink unimpeded through into the Earth, giving the whole body a weightedness while maintaining a suspended, light and sensitivity disposition and physical characteristic.
Thus, it is said that a true master is grounded even when balancing on a single toe. A silly anecdote, but expresses that a Master can ground despite being balanced on a toe because his entire mass is grounding through that toe and they are not held up or splinted by tension or excessive muscular contraction to maintain their balanced state.
In fact, in many ways, a Master who is immovable is expressing resistance, and this resistance is tension. In true Taoist principle, one must not resist change but instead flows with change like a river to the ocean, and while there are times to hold firm against pressure it is undoubtedly better to avoid such situations in the first place.