To stay in touch is to continue to write/text/call or otherwise stay in contact with someone.
It means to stay in communication; to continue to talk with them.
This is not an idiom - it means exactly what it says. You should stay fit and healthy.
Stay calm
An idiom is a saying or expression. There are many idioms that mean to stay away from, or avoid, someone. An example of such an idiom would be, "to steer clear of" someone.
This is not an idiom. It means just what it sounds like -- remain together in a group instead of separating.
Don't call me, I'll call you
It's not an idiom because it means just what it seems to mean. You should stay on the side of the fence that you are currently on and not climb over.
This is not an idiom - it means exactly what it says. You should stay fit and healthy.
Stay calm
An idiom is a saying or expression. There are many idioms that mean to stay away from, or avoid, someone. An example of such an idiom would be, "to steer clear of" someone.
"In touch" simply means to contact someone in some way - by calling them or writing or emailing or even going for a physical visit.
I'll get in touch with you later and we'll meet for dinner.
Someone who has the midas touch has a knack for making money.
If by touch it you mean simply place a finger on it, of course nothing will happen.
Don't call me, I'll call you
This is not an idiom. It means just what it sounds like -- remain together in a group instead of separating.
I keep in touch with my friends. If one of the my friends was going away, I would want to stay in touch with themKeep in touch = contact. Stay in touch = remain in contact.
It's not really an idiom. It means "what are you thinking about."