When you add books to Calibre, then depending on your settings and the circumstance there are times when the filename of the book file being added is used to determine information like the title and author for your Calibre book record.
IIRC Calibre comes with a default setting which expects books in a "title - author" format (i.e. anything after a spaced hyphen is considered as the author). This is specified as a regular expression pattern that you can see on the Preferences->Adding Books screen.
That particular pattern is "fine" for simple cases if your books are named in that way, but if the filenames include series information for instance then you will want to use a different pattern. One example of an additional pattern is included as the second plugin pre-bundled option mentioned above. This particular pattern will detect either "author - title" or "author - series - title" books.
Those two happen to be the two I use the most to use as a starting point. So this plugin lets a user quickly switch between their different patterns before adding a bunch of books. That saves on the manual "fiddling" that you often have to do where you get the title/author the wrong way around etc.
I hope that helps explain it. For more info see the help manual or ask in the Library Management forum.