The plugin.ini file is used to tell MAME which LUA plugins to run. You can find the file in C:\Mame folder.
To edit the file in Windows find the plugin.ini file in the C:\Mame folder. To open the file you may need to associate it with a text editor. To do this, simply right click it and select the "Open With..." option. Use Notepad if you don't have your own prefered editor. You can now double click the plugin.ini file and it will open.
Here is an example of a plugin.ini file...
There are only a few options in this file and they are all yes/no answers which are displayed as 0 for no and 1 for yes.
Finally, If you mess the whole file up, just delete it and run mame64 -cc from the command line to make another.