The value '100' is the variable. That is whatever your article ID is. This is in the article manager in the right side of the screen for each article. under 'ID'
to use the relative path it would be: index.php?option=com_content&view=article&id=100
If you are trying to link Articles you need to bring up the Article in your browser and copy everything after /index.php and use that info when you link (in TinyMCE).
If you want a simpler solution, I suggest using JCE (Joomla Content Editor) which has a handy linking feature that allows you to browse sections and Categories and link Articles without having to bring the Article up in your browser to get the path.
(Truly a life saver if you need to make lots of links between Articles or Modules.)