Everyone has a root. It is our connection to the ground beneath our feet.
The feeling of that connection can differ greatly. A person who carries themselves high up in the chest is more likely to have a weaker connection that a person who is relaxed, sunk, soft and aligned with the flow of gravity.
A strong root in Tai Chi allows a person to absorb a force down through their body to the ground without being unbalanced. It is a strong platform for returning or issuing force, for psycho/physical stability and ease of movement.
How to develop the root? It is important to relax the body and mind and soften. Some people visualise roots emerging from their feet or sacrum which go deep into the earth. Another image is a ball and chain, hanging from the base of the torso or a third leg ( a la Jake the Peg ).
John Kells taught a different approach. Instead of penetrating the ground, he allowed the ground to come up through him. To do this requires an accepting mind. Echoes of this can be found in the discipline of Eutony, where the performer/dancer feels the support of the earth through their bones and works with that feeling.
There are no quick results. Practise until it is part of your body/mind feeling.