What does `descendant` mean in XPATHNavigator's Compile function?

I've googled for few pages but failed to find the meaning of descendant word as typed out in the code below.

System::Xml::XPath::XPathExpression^ expr = nav->Compile("descendant::delivery[stockcode='ZMY201HR6US-A']");

Can someone tell me what descendant means?Thanks to anyone's helps.