Child theme creation and the use of the @import in the style css?

I am using the Opulus theme from nimbus themes and wanting to create a child theme. I was looking at some threads on the internet and from the reading of these threads I am not certain when to use or if I should use the @import url("../mainthemename/style.css");.

Could some please clarify on the use of when to use the @import or not to use it?

Thank you
Doug