The Insert After and Insert Before commands are enabled when a node in Grid View is selected. They add an element node at the same level, respectively, after and before the selected item. Change the name of the newly added element by double-clicking in its name cell and editing. Change the node type by clicking the element's icon (to the left of its name) and selecting the node type you want, or by using the command XML | Type.
See the topic XML | Document Structure for more information.