The Extend button prolongs an entity till it intersects with another one.
Quick Access Panel: 
Ribbon: Editor -> Instruments -> Extend
Menu: Tools -> Modify -> Extend
Select entities holding the Shift key and press Extend .
Example:
before: 
after: 
Go to CADEditorX
URL of this topic:
https://cadsofttools.de/help/cadeditorx/index.html?tools_menu_extend.htm