Probably you have already done this, but anyway here it goes: For a basic and visual explanation of the Multi-Editing feature, first off read Notepad++ User Manual.
Then, for more advanced examples involving the BetterMultiSelection plugin, please take a look at these problem-solving animations and follow the instructions to better grasp how the plugin works and enhances the standard Multi-Editing feature:
https://community.notepad-plus-plus.org/post/53667
https://community.notepad-plus-plus.org/post/54925
https://community.notepad-plus-plus.org/post/51906
https://community.notepad-plus-plus.org/post/53168 (explanation)
https://community.notepad-plus-plus.org/post/51915
https://community.notepad-plus-plus.org/post/51786
Take care and have fun!