PLR (Private Label Rights is content including articles, ebooks, software and more that you can rebrand and call your own.
Each PLR license is different, but they are all great!
PLR is used by many people for content creation, product creation etc. It can be sold as is and usually you can add your name as author.
PLR written content should be rewritten at least slightly to avoid duplicate content issues. However, software, templates etc are ready to be rebranded immediately and can be a great source of income.