What is the “M” button in the harvester section?

It lets you Merge a .txt file with your keyword list.  So if you wanted to pull a site: a list of urls, you can load the urls in the keyword section and then merge a file that contains site: with it.  It will add whatever is in the .txt file infront of any keywords in the list.

You can include multiple lines in the .txt file and it will take each keyword and add it to each line in the .txt file.