What does it mean when someone chews on their tongue ? nerves or something physical ?

It may be muscular: Some people that have a TMJ problem (joint in the wrong position) will rest their bite on their tongue. They do this to find a position where the joints are able to function somewhat better that biting their teeth together which may injure the joint. A good sign this may be occurring is if the joint clicks or pops where opening from biting the teeth together. Check with you dentist.

Nerves: Some questions are almost too foolish to answer. I do not know of anything physical that would cause tongue chewing other than grinding and clenching your teeth causing a scalloping of your tongue border making it easier to catch between your teeth.