how trade improves economic growth?

Answer 1

a couple thoughts...

when we trade with other countries it increases our national income
and
higher demand for our products means more jobs (you need manpower to fulfil orders)

Answer 2

Trade allows participants to specialize in what they do best and then exchange some of what they produce for other things they need

An interesting thing about trade is that even when one participant is better at producing everything, everyone can still be better off by trading. See http://en.wikipedia.org/wiki/Comparative_advantage for more information about that.