Being "grounded" is a term relating to how comfortable and connected one is wherever one is. How much one is facing the practical realities in front of one. How much one has one's legs planted on the solid ground, so to say.
We might touch many weird and unusual realities in session. We might expose people to completely different and sometimes fantastic viewpoints that they hadn't experienced before.
That sometimes lead to the person being blown out, spaced out, zipping around between these new viewpoints. Physical reality becomes kind of thin then, less dense and less real. That is not in itself bad, but there might be a lack of balance. She might feel lightheaded and spinning, with a need for holding on to something. She needs grounding.
Grounding is accomplished simply be bringing the person more in contact with the current surroundings. Asking her to come back and look around at the room, asking her what date it is, where she is, or whatever else is necessary.
Actually touching and interacting with the physical reality is grounding. Groundedness is largely a kinesthetic thing. It is feeling that one is there. Asking the person to walk around and touch physical objects is very useful as grounding.
Physical work is grounding. Working for an afternoon in the field is for sure grounding. Blue collar workers who do manual work are generally more grounded than white collar workers, and certainly more grounded than academic intellectuals who just deal with ideas about ideas. Grounding is about dealing with what is actually there without needing a lot of ideas about it.