XPath uses path expressions to select nodes in an XML/HTML document. The node is selected by following a path or steps.
The useful path expressions are listed below:
||Selects all nodes with the name "nodename"|
||Selects from the root node|
||Selects nodes in the document from the current node that matches the selection no matter where they are|
||Selects the current node|
||Selects the parent of the current node|
You can find examples and explanation here,