No I'm just talking about grounding in general.
Well, I am going to talk about your "foxhole radio" specifically, otherwise the subject is just too wide.
A foxhole radio is just a "Crystal Set" by any other name.
To work correctly, the radio needs an antenna, a length of wire which is as long and as high as possible.
It also needs and earth connection (ground connection).
The reason is that a small (very very small) current flows through the radio input circuit from the antenna to earth.
The better the earth connection, the more current will flow and the better the signal will sound.
If you do not have access to waterpipes, central heating radiators etc, you could use another length of wire similar to the antenna but running in the opposite direction from the antenna and connect that to the radio where the earth connection should go.
If you do that you have effectively made what is known as a dipole antenna.
If you want to try and use your foxhole radio outdoors (in a foxhole?) I suggest that you stick a short spike (6 to 12 inches long) into the ground (moist soil is best) and connect that to the ground connection on your radio.
A true story.
Some years ago I was out in the Scottish highlands and the guy with me was trying to listen to the cricket on the radio.
His radio was a little "Walkman" type thing and it just could not cut it.
So I got a length of wire out of the toolbox and strung it out and wrapped a few turns around the radio and then connected the end of the wire to a screwdriver which I stuck into the ground.
The guy was highly delighted to be able to listen to the cricket and I think he thought that I was some kind of magician/genius/miracle worker.
So, does this rambling missive help to clear your confusion?
JimB