PLR Stands for Private Label Rights.
When I got into mainstream I had no idea about this powerful phrase. Now I know that PLR content is similar to adult content. Except you buy ebooks, reports, articles etc. like you would images and video.
You are given certain rights with each product. Usually the rights to resell, rebrand and relabel the product. You are not usually allowed to resell the rights.
One of the things that has worked well for me is putting these books up on amazon using their createspace.com self publishing wing. That gives me lots of free search and affiliate traffic paying me a royalty on each sale.
Here is a free PDF mindmap I made on what you can do with PLR content. Clicking on the PDF itself will take you to a PLR bundle sale I am running if interested.
You can also search google for PLR content.
To get started I have some free PLR too.
http://freeplrstore.com/
Enjoy
Brad Gosse