1:-Want to know what does export command do??
What is its functionality?

The Best way to approach .. Go for the man page of "export" and "env" command.

2:- $at=1
$ echo $at
1
The variable above is it available to other script???

NO, It will be only for that terminal/shell. But if you want use that variable into your another scripts. You must have export it.

Thanks
Sanjay