Could you possibility tell me what difference is between these in meaning?
1.when she went to usa,she brought me some chockolate.
2.when she went to usa,she brought me back some chockolate.
The second one definitely means "She bought some chocolate in the USA, and brought it home to me.
In some cases, the first might have the same meaning as the second. In other cases, it might mean "She bought some chocolate in the USA, and then delivered it to me in the USA"
Also note that you would say "the USA" instead of "usa", and chockolate is misspelled (the proper spelling is "chocolate")
Hope this helps!
#1 could be read that you are in the US and she traveled to the US and brought you some chocolate.