What is grounding?

What is grounding?
Often mentioned, as in "Ground yourself in Tadasana (mountain pose)"
or as in "Frank seems well grounded" or "Casey has been grounded
after school for a week". What is the process and how do we get some?

Grounding, being grounded, is establishing a dialogue to the present
moment using our physical surroundings as the point of reference.
This sounds vaguely succinct, nevertheless there is this thing about
disconnect, of living too much in my head most of the time.
All that thinking takes me out of my physical awareness.

So then I have to make a conscious effort to reel in those windy
thoughts, bringing the attention back down to earth, and the
immediately accessible way is through my body's contact with the
ground. Grounding very literally is aware of touching the ground.