What Is The Meaning Of 'One Night Stand' ?

A one night stand is when you meet someone, possibly someone you've never met before and have a relationship with them with no intention on the part of either one or both parties to ever meet again or to continue the relationship. the time period is usually exactly what it says - for one whole or part of a night!
There are various reasons for one night stands - too much to drink, a party, trips away from home. they suit some people and as long as the two people involved are sensible and happy with the situation no harm is done. They can also be hurtful and end up with one or both people regretting what they've done. One of the very sad results of one night stands can be a child born as a result and the child may never know who their father is and that's heartbreaking.
Not really to be recommended but millions do it.

When a guy n girl have sex for one night and move their ways

I never had one but I know people who have. I guess its when you sleep with a person you met the same day meaning having sexual intercouse, then thats one... Usually, after a one night stand, you don't see each other again.

You fool around with someone and don't talk to them for a while after that

It is sleeping with a person with the intention not to do it again!

When  two partners meets  for  only one time  to sleep with one another and thats it

Try this--
. . . A sexual encounter with no - or at least no ostensibly agreed - preconception of an existing or incipient, ongoing, affection-based relationship . . .
Except where needed as an "escape" when one or the other participant has had "second" thoughts; or when one would hope to openly demean the other after the fact.

Sex with someone you just met