Extend

The Extend tool, on the Modify toolbar, is used to close gaps between lines in your drawing.  This may happen if you move part of your drawing and now have a gap where the lines used to touch.  This tool is most beneficial when you have multiple lines that you need to extend to another line.  You can select all the lines at once, and just click once on each of the lines that need to be extended.

If you only need to extend one line, it may be easier to select that line and use the grips to connect the ends of each line together.