Wah How to use "A While".

A while = A short time.
So if we say "I will be there in a while", it means "I will be there in a short time".

The opposite of a while would be a long time.

We can use a short while to emphasize the short period of time.

• It is impossible to master English in a while. (It is impossible to master English in a short time.)
• The system will go into operation in a short while. (The system will go into operation in a short time.)
• I'll wait here for a while. (I'll wait here for a short time.)
• Would you mind if I watch TV for a while?
• Let's sit here for a while and look at the view.
• Please stay out of my way for a short while.  
• This machine is out of order for a short while.
• Leave me alone for a short while. It's none of your business.

But Once in a while means sometimes, rarely, infrequently.

• They go to the movies together once in a while. (They sometimes go to the movies together.)
• He writes a letter once in a while but not so often.
• Father and I go fishing once in a while.
• My husband comes to see me once in a while.

I will be alright in a short while.